diff --git a/SourceGen/DisasmProject.cs b/SourceGen/DisasmProject.cs
index 7943c08..47c7ec5 100644
--- a/SourceGen/DisasmProject.cs
+++ b/SourceGen/DisasmProject.cs
@@ -1037,7 +1037,15 @@ namespace SourceGen {
             // order, so we can just overwrite earlier symbols with matching labels.
             foreach (PlatformSymbols ps in PlatformSyms) {
                 foreach (Symbol sym in ps) {
-                    SymbolTable[sym.Label] = sym;
+                    if (sym.Value == PlatformSymbols.ERASE_VALUE) {
+                        // "erase" value
+                        if (SymbolTable.TryGetValue(sym.Label, out Symbol found)) {
+                            SymbolTable.Remove(found);
+                        }
+                    } else {
+                        // overwrite
+                        SymbolTable[sym.Label] = sym;
+                    }
                 }
             }
 
diff --git a/SourceGen/PlatformSymbols.cs b/SourceGen/PlatformSymbols.cs
index 337e354..cba57e5 100644
--- a/SourceGen/PlatformSymbols.cs
+++ b/SourceGen/PlatformSymbols.cs
@@ -28,6 +28,8 @@ namespace SourceGen {
     /// </summary>
     public class PlatformSymbols : IEnumerable<Symbol> {
         public const string FILENAME_EXT = ".sym65";
+        private const string ERASE_VALUE_STR = "ERASE";
+        public const int ERASE_VALUE = -1;
         public static readonly string FILENAME_FILTER = Res.Strings.FILE_FILTER_SYM65;
 
         /// <summary>
@@ -165,20 +167,28 @@ namespace SourceGen {
                         } else if (typeAndDir == '>') {
                             direction = DefSymbol.DirectionFlags.Write;
                         }
+
                         string badParseMsg;
                         int value, numBase;
                         bool parseOk;
+                        string valueStr = matches[0].Groups[GROUP_VALUE].Value;
                         if (isConst) {
                             // Allow various numeric options, and preserve the value.
-                            parseOk = Asm65.Number.TryParseInt(matches[0].Groups[GROUP_VALUE].Value,
-                                out value, out numBase);
+                            parseOk = Asm65.Number.TryParseInt(valueStr, out value, out numBase);
                             badParseMsg =
                                 CommonUtil.Properties.Resources.ERR_INVALID_NUMERIC_CONSTANT;
+                        } else if (valueStr.ToUpperInvariant().Equals(ERASE_VALUE_STR)) {
+                            parseOk = true;
+                            value = ERASE_VALUE;
+                            numBase = 10;
+                            badParseMsg = CommonUtil.Properties.Resources.ERR_INVALID_ADDRESS;
                         } else {
                             // Allow things like "05/1000".  Always hex.
                             numBase = 16;
-                            parseOk = Asm65.Address.ParseAddress(matches[0].Groups[GROUP_VALUE].Value,
-                                (1 << 24) - 1, out value);
+                            parseOk = Asm65.Address.ParseAddress(valueStr, (1 << 24) - 1,
+                                out value);
+                            // limit to positive 24-bit values
+                            parseOk &= (value >= 0 && value < 0x01000000);
                             badParseMsg = CommonUtil.Properties.Resources.ERR_INVALID_ADDRESS;
                         }
 
diff --git a/SourceGen/RuntimeData/Apple/F8-ROM-nozp.sym65 b/SourceGen/RuntimeData/Apple/F8-ROM-nozp.sym65
new file mode 100644
index 0000000..c4a5b1c
--- /dev/null
+++ b/SourceGen/RuntimeData/Apple/F8-ROM-nozp.sym65
@@ -0,0 +1,45 @@
+; Copyright 2019 faddenSoft. All Rights Reserved.
+; See the LICENSE.txt file for distribution terms (Apache 2.0).
+
+;
+; Sometimes code uses the ROM entry points but redefines the zero-page
+; addresses for its own purposes.  Having the MON_* symbols is distracting.
+; This erases them from the symbol table.
+;
+; To use: include this file, and ensure it comes after F8-ROM.sym65 in
+; the platform symbol file list.
+;
+
+*SYNOPSIS Remove monitor ROM zero-page symbols
+
+MON_WNDLEFT     @ erase
+MON_WNDWDTH     @ erase
+MON_WNDTOP      @ erase
+MON_WNDBTM      @ erase
+MON_CH          @ erase
+MON_CV          @ erase
+MON_GBASL       @ erase
+MON_GBASH       @ erase
+MON_H2          @ erase
+MON_V2          @ erase
+MON_COLOR       @ erase
+MON_INVFLAG     @ erase
+MON_PROMPT      @ erase
+MON_CSWL        @ erase
+MON_CSWH        @ erase
+MON_KSWL        @ erase
+MON_KSWH        @ erase
+MON_PCL         @ erase
+MON_PCH         @ erase
+MON_A1L         @ erase
+MON_A1H         @ erase
+MON_A2L         @ erase
+MON_A2H         @ erase
+MON_A3L         @ erase
+MON_A3H         @ erase
+MON_A4L         @ erase
+MON_A4H         @ erase
+MON_A5L         @ erase
+MON_A5H         @ erase
+MON_RNDL        @ erase
+MON_RNDH        @ erase
diff --git a/SourceGen/RuntimeData/Help/advanced.html b/SourceGen/RuntimeData/Help/advanced.html
index d12731b..37a387e 100644
--- a/SourceGen/RuntimeData/Help/advanced.html
+++ b/SourceGen/RuntimeData/Help/advanced.html
@@ -71,7 +71,8 @@ written.</p>
 <p>The VALUE is a number in decimal, hexadecimal (with a leading '$'), or
 binary (with a leading '%').  The numeric base will be recorded and used when
 formatting the symbol in generated output, so use whichever form is most
-appropriate.  Values are unsigned 24-bit numbers.</p>
+appropriate.  Values are unsigned 24-bit numbers.  The special value
+"erase" may be used to erase a symbol defined in an earlier platform file.</p>
 
 <p>The WIDTH is optional, and ignored for constants.  It must be a
 decimal or hexadecimal value between 1 and 65536, inclusive.  If omitted,