There is some debate about peppermint actually deterring bees. Here is an article I read recently.
https://www.honeybeesuite.com/peppermint-confidential-the-preposterous-idea-that-mint-repels-bees/If you want to try it, I would not put the peppermint oil IN the nectar, as I'm not sure it would be safe for the hummers to ingest. Just take some oil and rub it on the feeder instead.
Something that often attracts bees to hummingbird feeders is if the feeder is filled with syrup that is too heavy. Hummingbird nectar should be 1:4 ratio of sugar to water. Hummers' bodies are used to metabolizing nectar, which is a very weak solution, but some people put more sugar in the solution in an effort to give the hummers more energy. If the syrup is a higher concentration though, it will attract bees. I have a hummingbird feeder out all summer, and I have for years, and I've never had the bees come and drink from it when it's filled with 1:4 syrup.
Something I have done however to get wasps away from the hummingbird feeder, which may work for bees too (as long as the syrup is weak) is to move the feeder to a different location, at least temporarily. Now that the bees know where the feeder is, they may keep coming back to it even if you change the syrup strength, but if you also change the location, they may view it as an entirely new food source (one that has never produced good forage) and leave it alone.