Temporary signature bans will never work.
They will just create an army of bot accounts that will make the job impossible for moderators, you can ban let's say 1000 accounts a day for 90 or 60 days, they will just create 91000 and once you're done with them the first ones will come back online.
Permanently removing the signature will probably mean for 99.999% the same as a ban.
I'm against the fee thing, it might serve a good cause but it just doesn't sound right.
So, just leave it as it is. Plagiarism=Permaban.
BTW I saw an identical message about temporary signature ban in some of users profiles:
Banned from displaying signatures until August 02, 2019, 07:05:09 PM
Theymos playing around as we speak?