Decide for yourself whether goblins are all evil. If they are, no problem.
If not, however, here'd be my suggestion:
-During the trial of the other goblins, let it come out that they're a fairly peaceful group of forest goblins who were captured and enslaved. Make them mostly sympathetic; have them promise to retreat to the forest if they're freed.
-Have one of them petition the courts for just and proper punishment of the cleric, who murdered a captive.
-Have the court deny the petition: the cleric goes free, with a warning not to repeat his actions.
-The goblin who petitioned the court approaches the cleric after being freed, and speaks a few words to the cleric: "You have killed my father, but he is not dead yet. Find me when you are ready."
-The murdered goblin become a ghost who haunts the cleric. Not necessarily a template-ghost, but rather your own ghostly creation, who can enter dreams, appear as a phantom figure, be generally scary. Eventually, the cleric will seek out the petitioning goblin, who will tell him how to atone for his evil.
Basically, turn this into an adventure. Don't rob the cleric of powers, because that feels like DM punishment of a PC; instead, give the cleric a chance at the spotlight, a chance to develop a character into something interesting.
Of course, if the player continues to be evil, the goblin ghost might get scarier; and eventually, the cleric might face censure from his church or even from his deity.
Daniel