I'd like to start by clarifying that this is not a question about whether custom signatures in posts should be allowed. That issue has been discussed before, and the SE policy on the matter is pretty clear:
Do not use signature, taglines, or greetings.
Every post you make is already “signed” with your standard user card, which links directly back to your user page. If you use an additional signature or tagline, it will be removed to reduce noise in the questions and answers.
Your user page belongs to you — fill it with information about your interests, links to stuff you’ve worked on, or whatever else you like!
It seems, however, that we've got several users, even quite established ones, here on math.SE that do append a lengthy custom signature to all their posts. I'm sure they don't do so out of malice, or a willful desire to flout the rules — most likely, they've just seen other users do it, and have copied the idea, having no idea that they're not supposed to do that here.
In the past, whenever I've noticed a user doing this, I've simply edited out the signature from the specific post and left a comment on the post linking to the policy and politely asking the user not to use such signatures any more, e.g. like this:
Hi, [username], and welcome to Mathematics Stack Exchange. I've edited your answer, in accordance with the guidelines given in our help center, to remove the extra "signature" you appended to it. Your posts here are already automatically signed with your username, avatar and a link to your user page, which is where you may include any extra information about yourself if you want.
(I do customize the comment as needed, and also make sure to upvote the answer after editing if it's any good; I've found that a bit of politeness and personal touch like that tends to make people a lot more receptive.)
As far as it goes, this approach seems to be successful, and I do plan to continue doing so.
The problem, however, is that this still leaves all the user's older posts unfixed. New users will keep seeing these posts, and getting the impression that padding one's answers with funny / quirky / personality-reflecting signatures is a normal and accepted custom here.
If it was just a few posts, editing them all would be no problem. But, for example, this user has, as of this writing, 534 answers, the vast majority of which (based on some quick sampling) appear to end with:
Hope this helps! Cheerio,
and as always,
I don't really feel like editing over 500 answers just to remove a repetitive signature, and I'm sure the user who posted them wouldn't appreciate that either, not to mention the way it would flood the front page. I suppose I could ask the user to edit their own answers themself, but I bet that would go down even worse — and I wouldn't blame them.
I could flag the situation for mod attention, but I'm pretty sure there's nothing they can do here that I can't. I do know that the SE devs have the technical ability to make mass edits, but I also know that they're reluctant to do it, perhaps for good reasons.
So I guess I'm just writing this to express a vague sense of discomfort and frustration with the situation, and to ask if anyone might have any practical suggestions on how to better handle such cases, or if I should just keep doing what I've done before.