Prayers can be dispelled after the cast, just as any spell. However, with a spell that is RiP, the opponent has a chance to dispel that prayer/spell in his own magic phase (using power dice), which is not allowed for a spell that lasts until the next magic phase of the caster.
Some think this is a huge advantage. I am not so sure, as dispelling a RiP spell in the enemy's own magic phase uses up at least some of his power dice and, of course, there is always the chance that he fails to dispel.