Feature Summary:

• Add an option to limit the length of a user signature (should be simple and fast).
• Alternatively, add an option for sysops to reset a user signature to default and block a specific user from changing it (kind of complicated and maybe a bit too forceful).
• Another option is to allow abuse filters to work separately on signatures (most universal and maybe more subtle).

Use Case(s):
This user has a ridiculously long signature and, aside from blocking him, we cannot really force him to shorten it:
https://pl.wikipedia.org/w/index.php?title=Dyskusja_wikipedysty:Azoniasty_220&diff=prev&oldid=73864865

Going through discussion diffs of that user is quite hard...

Benefits:
Making signatures more reasonable will improve the readability of discussion pages and diffs.

See also: T306574: Allow privileged user reset others signature