you can use autoenter with the textformat remove at the fields you want to replace the "original" value with the one without the formatting
If you want this function universally in your database, you can create a Custom Menu that replaces the Paste menu command with a script, that performs the "Paste" script step, with the "Paste without style" box checked. You'll need to revert to the standard Filemaker menu though whenever you edit the database structure, if you want to be able to paste into calculations, etc.
You can make a single script and call it via script trigger (on object exit or on object save)
The script would need:
Set Field By Name [ Get ( ActiveFieldTableName ) & "::" & Get ( ActiveFieldName ) ; TextFormatRemove ( Get ( ActiveFieldContents )) ]
The first part, Get ( ActiveFieldTableName ) & "::" & Get ( ActiveFieldName ) is specifies the target field name and the second (the calculated result) removes the formatting.