You can try upgrading them to a much larger tank (think 100 gallons or so--which is the recommended size for a single adult female RES, even before adding more turtles, anyway), and adding in a lot of hiding places/sight barriers (i.e. big fake plants) so they aren't always in each others' line of sight. If the male can't constantly see her, he may be less inclined to obsess over breeding.
That's the only thing I can think of that you can try. Otherwise, you'll have to entertain the idea of separating them permanently. Not that they'll care much--RES aren't exactly social!
On another note, do you WANT to breed them, or are you just wanting to house them together minus the breeding activity? RES are tragically common turtles, and finding proper homes for them is already incredibly difficult without adding another 150+ babies to the equation every year. There's no reason to have them together, in a situation where the female WILL be bred and likely produce eggs (which, as I'm sure you know, is hard on the female), if you're just going to dispose of the eggs/don't want to breed them, anyway. If your goal is to breed RES... Well, that's your choice, and, like I said, very large tank w/ sight barriers is your best bet... But I would implore you to seriously reconsider your decision to have a breeding project with this species.
Just my $.02.
Also, since it seems there are few people here on BD who have input on turtles, you might find it beneficial to find a good turtle forum or two to ask these questions on. There is a TurtleForum.com (run by the owner of Austin's Turtle Page, which is generally regarded as a good care reference). RedEarSlider.com has a forum, as well. TortoiseForum.org is primarily aimed at tortoises, but the community is fantastic (TBH, my favorite forum to browse and chat, lol) and you can still get a lot of great, knowledgeable/experienced opinions on the Aquatic Turtles forum.
