Flood the enclosure . Then they will float to the top . Then suck them up with a shop vac . Then spray them with wasp spray inside the vac . You may have to flood it a few times . Then let it dry out . Then put down diatomaceous earth . Make sure it's food grade not the stuff for pools .
Mary t, I've been testing the waters with the diatomaceous earth here just south of you...Literally testing the waters....Rain water.( It make the D/E useless.) Home Depot ONLINE sells FOOD GRADE in 4.4 pound bags. AS noted above, get the FOOD GRADE. Dump it in and mix it up with your substrate. Try it out once the weather turns dry. Also, don't leave any food behind and have a second pen or enclosure ready at all times in case of emergency so that you can fight your ant battles without bothering or harming your tortoise.
This is the time of year that red ants swarm and send their winged buddies off to start new colonies. Most of my red ant hills are in the driveway where no turtles or tortoises have access, so I use poison and sprays. If I find ants in any turtle/tortoise yards I try to isolate the hole, make a little mound around it like a volcano, and pour poison down into the hole. Then I cover it with a piece of plywood and set a brick on it so the turtles/tortoises can't get to it.
Do a search for ant remedies. We've talked a lot about it here on the forum and there are several very good threads/remedies here.