7 years and no eggs?
Do you mean the eggs have never hatched, or she's never produced eggs, or you've never found them?
Do you know how old the turtles are? The female may be "underage".
What is the size of the pen? Is there a suitable place for the female to dig a nest? She may be laying eggs on the surface and they're being eaten before you find them.
If they are sharing small quarters and cannot avoid eachother, then it might not be unusual that they become more agressive.
I have seen one male mount another, have his feet get clamped in, so that it appears that mating is occuring when in fact he's merely stuck.
You haven't described any physical differences between the two turtles: can you tell us what leads you to believe that they are male and female? If your vet isn't a herp vet with box turtle experience, he/she may be making assumptions about gender based on what you say about "mating" taking place, which may not be whats happening.
Stephanie