Ants don't like cloves
For the last 4 years I have been putting cloves in drawers and cupboards and in other nooks and crannies in my caravan home.
No ants!
Cheap and simple
Any ants that do try to get in are usually through the roof but they soon meet their doom
Also an Oil of Cloves solution wiped over surfaces stops mould. - Works wonders on my awning as well.