They're one of the clover mites or velvet mites. I'd think that an occasional spray with soapy water (or insecticidal soap) would help.

Velvet mites, btw, are harmless to humans and are predators -- highly beneficial.

If they are clover mites then they are again harmless to humans, but feed on grass and clover.

If your mites seem to scurry around like someone desperately trying to find their car keys, then you most likely have predacious (beneficial) mites.