hormones what can i do (bitch, due in season, mounting other dog)

Dora is due to come in to season mid next month but in last 3 days shes started to hump misty it started of just once or twice but no shes doing about 20 times a day to him only him i dont know how much more he can take of it has anyone got any ideas of what i might be able to do ??
heres a pic i took so u can see

not great but please if anyone can help

My Belgium shepherd has been doing this to her best friend, a labrador, since she was a pup. And the lab (also f/male) to her. We don't take too much notice, just break them up and say Noooooo, but it still goes on. I believe its a way of showing their dominance over the other dog.

she has only done it when she due to come in season + because normally he gets to go home its not a problem then when her seasons over shes fine again