Use "Combat" insect killer (http://www.combatbugs.com). We use it in our household. Used to to have ants in flower pots, living in the dining tables' legs, in the cupboard, and outside the kitchen window, even in the false ceiling. Leave a few of the traps near where the ant trails and they will go in, come in contact with the poison, and take it back to their nest. In a few days or a week's time all the ants will have been poisoned. It will get rid of cockroaches too (my old workplace had cockroaches, but I kept a few of the traps underneath my desk so my area was roach-free). As is with most insecticides, keep it out of reach of little kids. Also, the poison/bait inside the little plastic houses gets mouldy after a couple of weeks, especially in this damp monsoon weather, so either use up the whole pack, or seal unused traps in an airtight container. Also remember to throw away old traps after maybe a couple of weeks, because the uneaten poison/bait inside tends to liquify after some time - it may ruin surfaces which can be stained.