I've been wondering the same thing. When I paste an email address, for example from Mail into my contacts db, it comes in with formatting. I have found that doing a Cmd-Z (undo) right away takes away the formatting, but I don't know how to automate that for users. Maybe with FMP 10's new script triggers(?), but we are using v9.
a simple calculation:
TextFormatRemove (Self )
placed into the auto-enter option ( always evaluate ) for that field will solve your problems.
( whichever format has the inserted text it will revert back to the format assigned, in layout mode, to that field )