Someone mentioned "boronic acid". I've used orthoboric acid to good effect.
I buy it by the pound squeeze bottle and puff it into any crack that I see
ants coming through (and some that I don't). It's apparently harmless to
humans if you don't eat it or wipe it in your eyes. Children and pets don't
know not to do this, so you have to sweep it all into the cracks. I don't
have children or pets, so my house sometimes looks like an explosion in a
baby powder factory. I've taken to putting it on my finger and spreading
it in the cracks -- saves on boric acid and airborne particles.
The ants don't come past the stuff I put down, but they keep finding new
cracks. No surprise -- what else do they have to do with their time,
especially when the alternative is drowning (they mostly try to get in
when it rains)?
BTW -- ants may or may not like flour, but other bugs (like cockroaches)
love it. Roaches, on the other hand, also succumb to boric acid.
Re William Kitchen's comment on salt: I don't think salt would faze ants
at all -- they have exoskeletons, unlike slugs.
Kennita
Kennita Watson | The bond that links your true family is not one of blood,
kwatson@netcom.com| but of respect and joy in each other's life. Rarely do
| members of the same family grow up under the same roof.
| -- Richard Bach, _Illusions_