If they insist on having help, then let them have it- just be sure you divide the XP (if the PC-side wins) amongst all comers, PC and NPC alike- just as Crothian suggested. Those 1st-level Warrior guards (who survive) will be shooting up in the levels pretty quickly, while the PCs are stuck in neutral.
More importantly,
up the power of the BBEG and his henchmen- spontaneously, behind-the-scenes- so the guards are either useless, or occupied with the henchmen and can't help the PCs at all against the BBEG himself. Then the PCs will be forced into a showdown with the BBEG, whether they want it or not; if they avoid said showdown, then the BBEG just wins. Game over, they lose. IME, most players hate to lose, and will do what they have to in such a situation even if their helpful city guards aren't any real help after all.
You could also give the BBEG and/or one or more henchmen some good area-effect attack spells, or even just loads of Alchemist's Fire. Having an entire cadre of city guards with the party means lots of people crammed together in tight spaces- there's no avoiding that, since going in in small groups really just means doing it the way the PCs would have done it if going in by themselves (except that the guards are considerably less competent and powerful, and thus most likely to be slaughtered). Take advantage of the grouped-together allies and blow things up. Once the players see that the guards are basically useless, they'll stop looking for help and do things "the right way-" meaning, on their own.
And if you use the area-affect trick, if the PCs ever try the "let's hire some help" thing again- the guards will reject them out of hand, immediately, and they'll have an excellent reason for doing so. "What! Get thirty
more men killed supporting your glorious attack? What kind of idiots do you take us for? Handle it yourselves the way you should have last time!"
EDIT: Ooo, Black Swan's idea is a good one.

BBEGs buy off the local authorities all the time in heroic fiction, it's part of why the heroes have to step up to the plate.