I'm interested if there's a way to blacklist a URL so that it is removed if someone attempts to put a blacklisted link in their signature, for example those dragon egg pictures. If not, would it be easy to manually plug in? I ask because the last thing I want to do is manually enforce such a rule.
The Help Manual's spam section only specifies removing threads.
edit: After further research, I came upon $pattern and $replace, would this work? Would it be easy to implement in the signature database?
Censoring is not covered in detail in the manual as it's relatively straight forward. You can see it in the options section of the admin CP (in a demo).
It doesn't work exactly like you're suggesting there, as it won't "see" the BB code itself. You'd just match against "dragcave" and censor it. It doesn't necessarily remove the whole section, but the link won't function.