There is no legitimate way. To say that something like a few wishes (which are a nearly unlimited resource for an efreet at 3/day every day for its entire life, none of which is can use for itself) has any relevant cost or danger is stretching it. To say that the demand is impossible or unreasonable makes ANY demand of any planar bound creature impossible or unreasonable, where you might as well have just found a hostile creature and convinced it to help you, rather than use the only spell printed that allows you to purposely extract service from outsiders.
The best thing to do is make a house rule or three, but give the players a few wishes since they didn't break any rules to get them.
1. You could change efreets to limited wish rather than wish. That actually solves a lot of problems with the amount of wishing that would probably be going on in the plane of fire.
2. Give planar binding some material component costs, sort of like planar ally. Instead of giving the money to the creature, the amount of material component spent determines how long the binding lasts (minutes, hours, or days+).
3. House rule that any ongoing spell effects end when the called creature returns home. House rule that any equipment the called creature had when it arrived disappears when the creature dies or returns home. That at least means players can't bind outsiders with magic weapons and take them, or get permanent spells got free. Not every use of planar binding is an abuse, it's fair to give them some utility out of the spells.
The best thing to do is make a house rule or three, but give the players a few wishes since they didn't break any rules to get them.
1. You could change efreets to limited wish rather than wish. That actually solves a lot of problems with the amount of wishing that would probably be going on in the plane of fire.
2. Give planar binding some material component costs, sort of like planar ally. Instead of giving the money to the creature, the amount of material component spent determines how long the binding lasts (minutes, hours, or days+).
3. House rule that any ongoing spell effects end when the called creature returns home. House rule that any equipment the called creature had when it arrived disappears when the creature dies or returns home. That at least means players can't bind outsiders with magic weapons and take them, or get permanent spells got free. Not every use of planar binding is an abuse, it's fair to give them some utility out of the spells.