How can I stop creeper griefing without changing the gamerule?
Here is what worked with some tests on a vanilla server and in single player. Place this in a command block and run it on a clock. This creeper does not do player or block damage.
/entitydata @e[type=Creeper] {ExplosionRadius:0}
Creepers will stay away from ocelots. So station ocelots (on leash) around your area