Goons Are Great posted:

The absolute top priority here is to remove their motivation to come in in the first place. Even the best poison is useless if they are starving and find food inside. Try to find what they are looking for and remove it, seal it, clean it etc. Ants will literally move mountains to get what they want if they need it.
If they enter in notable numbers (beyond, say, 20 ants at the same time tops, usually way less for most species), they do have a reason for that, even if seems impossible.

To locate this, it's best to follow the tracks they build, observe them closely, try for a while not to disturb them to see their natural route they are taking. Pick one specific ant and follow it where ever it goes for starters.
If you want to be really good or if it's really difficult to keep track, offer them a tiny bit of honey or sugar water and color that with food coloring in a color that does not fit at all to their exoskeleton (fluorescent stuff works!). This color will then start shining through their abdomen after eating, making tracking easier. Since the ants will share the food you offered with other ants, this way you can "infect" an entire colony with color, without hurting them.

Once you removed whatever they are after, you can get started on removing the means of them coming in, too. First off, simply shoo away the ants still walking around, just trampling will do. Then clean their pheromone tracks with (if the floor allows) hot water mixed with vinegar based cleaning agents. Clean it from start to finish, else they can find the ends of the route again. If necessary, you can brush up the entire floor they're infesting this way, it will certainly repel them for a bit due to the stinking nature of vinegar.
Afterwards you can seal the entrances. If it's carpenter ants, this might be difficult depending on where it is, as they can chew through a lot. Wood, plastic, silicone based stuff, they could get through. The point however is that they should not have a reason to do so - putting in effort in chewing through for nothing is a pointless endeavor they won't take.

The last and maybe also the first step to ensure they not only have no reason to get in, but also a reason to stay out, is to offer them some food outside, if this is possible for you based on your living situation. Some landlords or neighbors don't like to see ants being fed, if anyone nearby is afraid of ants or insects etc this might also be a problem, but I can assure you that it will not make their population explode or cause a whole new infestation, it's just a way to motivate them to take this easier food rather than taking the risk of entering your potentially dangerous home. I'm not talking about liters of honey, depending on their numbers a simple bottle top filled with some honey, a small plate with a few milliliters, some you can pour it into works fine. They don't need much.

For most ant species at your place this should be more than enough to make them go away. Carpenter ants are not a pest species and usually easy to get rid of. All of this can be done in a matter of a few hours or maybe a day or two (if you're coloring them, as they need a while to eat), costs you nearly nothing and should be effective. Also you avoid the biochemical horror that is widespread ant poison for your entire local fauna.