I'm having trouble envisioning how termites would get into a plastic bin. Most termites live in soil, and tunnel into woody things to get their food. If the plastic bin has a bottom, and no holes, then how are the termites getting in ??

If they're tunneling in then just move the bin and make sure the bottom is secure.

If they're living in it, dump it all out and spread it out to dry.

Are you sure they're termites? Ants often make their nests in compost piles that are left unattended. Ants have a distinct 'wasp waist', where termites have no waist at all.

Bet on ants - spring winged getting ready to migrate. Unless there is alot of fresh wood in your plastic bin there is nothing to appeal to termites.

As already mentioned, you don't need to treat - just dry it out either with the addition of lots of browns, increase air circulation, or spread and let dry.

Dave

As a rule termites will be whiteish in color and will not have a pinched waist while ants can be any of several colors but will have that punched waist. Often I will find, in the really dry parts of the compost bin, an ants best and the larva will be a whiteish color with no pinched waist and they will not, yet, look like ants and certainly do not look like termites to someone that has seen both.
Determine why they might be there and correct that and they will move on. If the material is too dry ants could nest there, but even in a too dry mix that is turned often will also discourage them. Other than getting your mix properly working there is really nothing more than that you need to do.

reg_pnw7, I think you're misunderstanding how plastic bin composters work. Generally, they are open to the ground, somehow, which lets in worms and soil microbes (and ants or termites, should they be inclined), and lets excess moisture drain out. I believe the commercial bins are completely open on the bottom. When building your own bin out of a garbage can or the like, it's usually recommended to drill several holes in the bottom, for all of the same reasons.