That kind of thing is controlled by the forum software. To implement a feature like that is to actually dig into the source code for forum itself which is something you really don't want to mess around with. It's not as simple as just copy-and-pasting.
have a plug-in system. If any enterprising programmer wants to take a stab at creating a plugin that allows people to ignore specific users' sigs, have at it.
But please keep in mind that such a feature is probably not worth implementing in the first place
. If a signature just annoys you, suck it up and deal with it
. If a signature is actually offensive, then it should be changed, in which case you should talk to a mod about it. But stuff like the Nohbody's sig, while potentially annoying to some people, isn't offensive and is perfectly fine as far as sigs go.