In reply to Chesher cat: naming the company sounds like a bad idea to me. There is no one way to stop bullying, if there was it wouldn't be such a problem.
Imo taking things higher up (teachers/employers) rarely helps, unless you have someone amazing in HR or working above you. Bullies are generally insecure and other people suffering, failing, getting angry, being unhappy or appearing weaker than them gives them the self esteem boost they're looking for. Bullying is also usually insidious, lots of little things that add up and no one thing that's easy to point to and say "this is how I'm being bullied". Formal complaints will often just show them they're doing really well, they'll pass it off ad overreacting but you'll both know they've won, it'll die down for a bit and come back. Unless you think a complaint will get them fired I'd avoid it, but that's just my opinion and I don't know what's going on with you.
It's easier to stop if you spot it early, by not reacting, but once you've been pinned as a target I think it goes in cycles and it's hard to stop it creeping back. In all honesty if it gets this far your options are 1) get out (new job), 2) never react and wait till they move on (hard to do and unlikely to work imo), 3) confide in a mutual friend who'll call them out instead of ignoring it when you're not around (and thereby tacitly enabling/encouraging it), having been the mutual acquaintence this does work in the short term but I imagine it could spectacularly backfire if you choose your friend badly and may also start up again later on. 4) The other option is to confront it, remembering theyre the weak ones and they havnt got to you, you're just reacting because they're a petty annoyance and you've got bigger fish to fry. This is my biased advice by the way but don't treat them like your the victim and don't for ask them to stop bullying you because they're getting to you. I think if you're like me and generally non-confrontational and easy going a raised voice and a hard stare in private is pretty effective. Just stay calm, in control and tell them to stop bothering you, they're annoying you and you don't have time for it. If your both male then a strong tone can convey a subtle threat of physical violence which is quite powerful, especially if they've never seen that side of you before. Bullies are usually cowards, provided it's in private and they're not losing face they'll probably back of rather than risk losing out. Like I said, "usually cowards", there are plenty bullies who love a good scrap as well abd you want to end things, not escalate them.
I don't know how helpful that is to you without knowing exactly what's going on but fwiw I'd avoid doing anything subtle in retribution, you'll only exacerbate things and quite possibly lose