We tend to use this:
Amazon.co.uk User Recommendation
on the outside of the house where we think the ants might get in ie, around doors, air-bricks etc.
Half my post disappeared.
We use this
Amazon.co.uk User Recommendation
You can get small plastic tubs, about the size of a shoe-polish tin, which you put it in and leave it near the ant run. The tubs have lids, so they're safe to use if you have pets.
An ad hoc remedy is to put an opened can of coke or other soft drink by the entrance. Attracted by the smell of sugar they crawl up the can and drop inside, unable to get out again.Make sure you don't inadvertently pick up the can and drink it though, as happened to one of my pupils on a trip to Normandy.
The stuff in bhg481’s post is not just an ant powder type remedy – but apparently the ants take the stuff back to the nest and feed it to the queen. Once the queen is dead the colony will die.
Support bhg and Hymie. Ants were always a problem in France. The supermarkets sold 'ant hotels'. They are very effective, sure you can get them here. :)