Why do you want spammers to be able to choose the fields they want? They'll want a dozen spam links in a webpage field. Some idiot child will eventually create a "last time I had sex with your mother" field and set it to 5 minutes.
It's a BAD idea to let members choose their own custom fields, it'll cause too many problems...
This can happen everywhere: in PM, in signature, in public posts subject or body, and in admin custom field too.
The solution? Rigid forum/website rules again those behaviors
: if a subject goes wrong for heavy things like these
, bann it or delete his account... solved the problem...
I think that every admin would to have perfect users for its website or forum... but it is not possible: spammers or mannerless or bad educated people are everywhere.
But this doesn't mean that if a think is ok for other people cannot be implemented cause mannerless people: for serious bad behaviors
I usually ban or delete their account. End of game.