Prevention
A few ants around the house are actually helpful. They prey on flea larvae and the young of other pests, such as silverfish and clothes moths. In addition, they clean up tiny particles of organic debris, which would feed other pests. To keep from having too many:

• Block the entrances. Caulk cracks.
• Do not leave food for them. Sweep, wash dishes and use a garbage can with a tight fitting lid.
• Store food in glass jars with rubber seals or gaskets attached to the lids (ants and other stored-food pests can travel up the threads of a screw top jar) or store in plastic containers with tight-fitting snap-on lids.
• Remove food and water sources. Wipe cabinets and counters with a mixture of one part water and one part vinegar. Caution: Can lighten wood surfaces.

Treatment - Stop 'em in their tracks

For indoor counter surfaces, use clear vinegar and a sponge.  Wipe the counters, particularly along the walls and backsplashes, to disorient the ants.  They need the smell of food to retrace their steps back to the mound.  Vinegar interrupts this process and ants go elsewhere.

If you find ants indoors, follow their trail back to their point of entry. Mark that spot. Use a spray bottle of vinegar and water or a soapy sponge to wipe them up. Block the entrance with a piece of tape. Leave it there until you can get around to sealing the hole permanently. When you get around to it, use silicone caulk for an effective seal. If you are contending with a large indoor infestation of ants, try a boric acid bait.

See Materials for Boric Acid Bait.