I've got ants!

Northern California black ants. The kind that invade whenever the weather
get really hot or really wet. The kind that ignore food. They're looking
for something (aren't we all), but it's not food. Maybe the weather changes
disrupt their homes (like earthquakes on ant-scale).

Anyway, lately they've been swarming the top edges of the hot tub. The hot
tub sits on a gravel bed surrounded by a wood frame with dirt or grass.
It's going to foggy with occasional periods of rain until next summer. What
do you think my best ant-kill and return-prevention options are?