How to Prevent Mints from Crossing

I purchased mint plants from Richters and noticed a couple have started to cross. The plants haven't flowered and I divide all the runners out. I wanted to know if there is anything I can do to prevent plants from crossing. The plants that seemed to do this are: wintergreen mint, pear mint and candy pop mint. I can't keep them too far from each other because it isn't practical. If you can let me know how you manage to keep mint from crossing at your farm please let me know.

I presume by "crossing" you mean that the plants grow into each other. They cannot produce hybrids from germinating seeds unless they produce fertile flowers and seeds from those. Many of the tasty hybrids are sterile and never set seed.
We keep the plants in separate pots and keep enough distance between different types of mints, so that we catch them before they have a chance to invade each others pots!
In the garden I always advise people to purchase a big plastic garbage can for EACH mint, take out the bottom to provide excellent drainage in clay soil or punch some holes near the bottom for sandy soil and bury the cans far enough so that only 5-10cm sticks out of the soil. It is a bear of a job, but well worth the effort! Plant your mints into those "cages" and now you only have to catch any runners that try to crawl over the rim. By having the rim stick up 10cm, you have plenty of notice of their evil intent and can cut them off.