Office Ants

Sometimes, even three stories up, you run across the problem of ants.  Ants of course live in dirt and when you have many plants sometimes they get the idea that it would make a nice home. Apart from removing the dirt you have few options.  What I recommend is taking the plant out and soaking the dirt to drive the ants out. After you are sure the ants are out and have fled for their lives you can then let the water drain out and put it back where it came from.
To prevent ants from returning you need to try to find out what they've been eating that makes the dirt such a convenient home. In my office's case it seems to be crumbs left over from having food in the conference room a little too often. Once the food source is removed all you have left to do is a little preventative spraying and you should be fine. If they return then something else is wrong. Maybe the floor under or over you is providing plenty of food for everyone and the problem you're having may just be run over from theirs. In that case you would need to discuss this with your neighbors.