Sounds like a great idea and compromise. It has my support. How about an 'ignore this user's signature' button in a similar way you can ignore users?

Or maybe certain staff or admins could have the choice of disabling certain user's signatures as punishment instead of a ban. The option to either disable them for a short period of a week or something or for three-time offenders disabling them forever (or something extreme like several months even up to a year) may be a greater deterrent.