Okay, I have a solution that seems to work for this. I remapped Ctrl-V to use a script like this, where I don't want the user to be able to paste into field1, field2, etc. :
Set Variable [$Field; Value:Get ( ActiveFieldTableName ) & "::" & Get ( ActiveFieldName )]
If [$Field = GetFieldName ( field1 ) or GetFieldName ( field2 ) or ...]
Show Custom Dialog ["Paste not allowed for this field."]
Exit Script 
Paste [No style]
Is there a way to get a fully resolved Active Field Name without first grabbing its Table Name and then grabbing its Field Name?