Disable Creepers

You can set the creeper’s “ExplosionRadius” value to 0 to prevent both block damage and heart damage. Example using 1.9 mechanics:

/entitydata @e[type=Creeper,tag=!processed] {ExplosionRadius:0,Tags:["processed"]}

 
If you mean to prevent them from exploding to begin with, then you have several options depending on what you’re looking for. If you want creepers to still track you, you can set their “Fuse” tag to maximum so that it takes ~27 minutes for it to explode (the timer resets when it gets out of explosion range):

/entitydata @e[type=Creeper,tag=!processed] {Fuse:32767,Tags:["processed"]}

 
If you don’t want creepers to track you to begin with, you can set their “generic.followRange” value to 0:

/entitydata @e[type=Creeper,tag=!processed] {Attributes:[{Name:"generic.followRange",Base:0.0}],Tags:["processed"]}