My dog (mixed breed, 40 pounds, spayed) is often the target of "humpers".

I don't know what it is about my dog but in off-leash parks, other dogs often try to hump her. She's 9 months old and was spayed about 3 months ago. Is she still giving off a hormonal scent? I have a second older dog (10 years) and this never happens to her.

What should be my reaction when this happens? I don't want to grab the other dog for fear of getting bitten. Is this just normal dog behavior that should be tolerated?