Stating the obvious.... use a bigger pot. And pull out clumps and refresh the soil if it gets crowded.

I grow mint in a container and always have. I use a squat 5G and it's good for at least 2-3 seasons before I need to divide and replant.

You could consider growing in a bottomless container sunk into the ground. Any rhizomes stay within ~30cm of the surface, so the roots will enjoy unrestricted depth without you worrying about the rhizomes spreading.

What size pot are you using?

Be careful with those bottomless containers!! I have had mint rhizomes escape through the drainage holes of the bottom of just a regular container sitting on the ground and root out into the surrounding soil. Make sure your container is deep enough to restrict this sort of migration..........