Ants apparently won't cross a line of chalk / powder. If you use a standard block of chalk for a black board you can draw a nice solid line on the ground and it will stop them completely in their tracks.
You have to make sure that there isn't any gaps in the chalk line as they will find it and go through it but so long as your line is solid they'll be kept at bay. You can also use a powder in the place of the chalk which will have the same effect (baking powder tends to work well)
Another option is you can apply double sided sticky tape, or you can get special tape designed for stopping ants and other critters and this will literally stop them in there tracks and they will be stuck there until you decide to do what you will with them - gives you the nice sense of being God in the world of ants.
On a side note - if spiders are also a problem, if you know where they come in (cracks in the wall or doors) if you place a nice pile of conkers in the way they'll be deterred from coming in. Works and can look like some decorations if you're into that natural decor type stuff.