In this kind of thing you just go full on formal by requiring the request via e-mail, were you notify them that you're not qualified to do it and require confirmation (and, if applicable, confirmation that your own manager authorizes it).
(Also if you are busy with some other project, be very very explicit it will have to be put on hold and request confirmation that the manager in charge of that project has authorized it).
By this point, in all likelihood the person doing the request will give up. If not and you do get a go ahead, you're now fully covered to take tons of time, do a bad job of it and it's will officially be the fault of the person who asked you to do it.