[Railstar]

I would treat it as Vow [-5], similar to the Vow "never use black magic" on the Freelance Wizard template in GURPS Banestorm.

Technically a vow to never use "black magic" doesn't cost you anything if you're not interested in using black magic... and there's very good reasons not to use black magic.

It prohibits a range of other magic items, but probably not most magic items. I'd be fairly consistent with the -5, because if magic items are common then the cleric will find more items he cannot use, but if magic items are rare then not being able to use one of the few magic items found in the campaign will be a greater loss.

Also, I consider taking such a disadvantage to be an agreement between the PC & GM saying, "I expect some of the magic items we find to be unusable by me."

Never saying the name of other gods would be the Forbidden Utterance quirk.