I think you’re in a warmer Zone than me. I’ve never had Spider Mites on my Marigolds up here (Zone 5a) and they’re supposedly a ‘sacrificial plant’ that is used to lure Spider Mites in to keep them off of other plants, mainly tomatoes.
However, here are some things they say to do if you get them:
https://seedsandgrain.com/spider-mites-on-marigolds/
Borage warning, unless you have tons of room.