I know of no repellant, be be aware of what you may be doing to attract them. Think of what they look for for food.. sweet smelling flowers.... avoid scented lotions, deoderants and perfumes, for a start. I would also avoid brightly colored clothing that might resemble a flower's colors.
And, having learned the hard way from my 2 yr old one morning many years ago, don't walk through the grass early in the morning when the dew is still on the grass. (He ran around on it one morning in sandals as I was putting a piece of mail in the mailbox). The dew on the grass attracts them and you don't realize they are there, because they are not flying around, they are "lit" on the blades of grass drinking from the tiny water droplets... you can step on them without realizing!
Avoid leaving sweet things like empty soda cans/bottles outside... unless you have thoroughly rinsed them out. The sugary stuff inside attracts them.