A solution that may have been proposed already might be losing progression points, a percentage of what said player has. If that isn't enough, and they continue to abuse it, it might be a good idea to have the progression points removed with increased percentage the more they die, as well as a HUGE amount of those points being removed for suicides, as that shouldn't be taken lightly and the "easy" way out really shouldn't be so easy..
Getting out of the DR is easy, but what if it people who die and come back, don't exactly "come back". Instead of being whole, they become a "spirit" of sorts, requiring them to say, do a quest, to physically return to the realm of the living. This could also give room for hauntings and such in RP, but taking a decent chunk of energy from the "soul" when they talk (seeing as it wouldn't be easy for the dead to communication with the living.) Nor would they be able to attack or interact with objects (If someone isn't actually "there" they shouldn't be able to do anything)