The guys I play with never lose objectives to back capping. We stop it in three ways, listed below in descending order of priority:
Don't lose the objective you're currently defending. This is 100% guaranteed to stop all back capping attempts cold. Bonus: there's a good chance the last man standing will be a back-capper, helplessly waiting for death in the restricted zone as your team hunts him down and humiliates him. His team will likely hate him. I've seen it plenty of times, and it's hilarious.
Post defenders at the wide flanks. It just takes one on each side. Not only will it do lots to stop back-cappers from reaching the next objective, it also helps stop wide flanks of the current objective. Further, if positioned correctly, your wide flankers will be able to pick off enemy attackers heading for the current objective. It's a win-win.
Each time your team spawns a new wave, do a quick sweep of the next objective on the way to the current objective. It should be near, if not at, your current spawn point. Not everyone in the wave has to do it, just one or two. Bonus: if you find a back-capper, he can't shoot back in the restricted zone--free kill. Hilarity ensues.
Points 2 and 3 also deter further attempts at back capping. Once the attacking team gets caught trying to back cap, they know your team is watching for it and they typically stop trying to do it as the the risk is just too high.
Also remember that if a team is back capping, they are a player short (at least one, but as many as they have on the next objective). It doesn't sound like much but it can definitely make a difference when it starts getting down to the wire. Then you see those guys waiting on the next objective frantically trying to make it back to the current objective without getting killed.
Bonus tip: Don't fall into the trap so many defending players fall into. When you lose a point, leave it and forget it. Sprint immediately to the next point. Doing so generally gets you there before a back-capper can finish capping. Do NOT loiter around the point you just lost trying to get kills. You're absolutely going to die and leave your team shorter than it has to be as they defend the next point.