How to respond to a citation request which is not relevant?

Opinions vary, but I think it is very important to acknowledge prior art, that is, priority. To leave important prior (strongly related) work out of a bibliography is to communicate the idea that there was no prior work. Indeed, one can "cultivate" lack of references to one's competitors by carefully not looking at their work, thus, seemingly having no obligation to cite them (or otherwise acknowledge their existence!?!) But this is inappropriate, as it would convey a false impression to your readers, as well as being unfair to your competitors or predecessors. That is, it's not only what you use or rely on, but works that came before yours.

If you get requests for citations to irrelevant publications, just ignore them. Seriously, that's all you need to do. I get suggestions/requests for citations all the time. Sometimes they are relevant things that I overlooked (in which case, I add them), but usually they are not.

I don't even usually bother writing back to the people sending the requests. It's rude of them to ask you to cite their irrelevant work, and so I feel I am under no obligation to write back with a polite demurral. The people grubbing around for these citations are extremely unlikely to be leaders in the field (the kind of people who might makes things difficult for you if they think you are being rude to them); the real leaders do not need to waste their time drumming up undeserved citations.