You'll need to identify the pest properly before deciding what kind of control measure to use. Are these 'bugs' existing in a totally empty greenhouse (no plants)? If so, for how long?

Perhaps you can take some pictures of the little creatures, or (at the very least) try to describe them and their behavior more thoroughly.

Whatever you do, don't set off a bomb that has been manufactured for home use. It could damage the glazing material of the greenhouse (just for one reason), and possibly ignite the heating source (if there is one).

As stated above the critters need to be positively identified and the best treatment IMHO (if a pest) is natural predators. Using chemicals can and will throw off the balance and you may end up with more pests and without nature on your side. Chemicals will kill off natural predators also and that's not good.

Precious few natural predators reside in a greenhouse.

PRECIOUS being the key word. They have the means to multiply if given the chance. Also they are still natural pest control even if released by man. Just not natural in occurrence.