What is the ruling of Islamic law regarding one who describes Shaykh al-Islam Ibn Taymiyyah as a saint (qiddīs), or writes "May Allah sanctify his soul" beside his name?

There is a difference between supplication (du'a') and bestowing titles. The saying "May Allah sanctify the soul of so-and-so" is a supplication for blessings and purity upon them, and there is no harm in it. It has even been used by some of the People of Sunnah. As for bestowing the title "saint" (qaddis), it is not a supplication. Rather, it is a title used by Christians for their rabbis and monks, and we have been commanded to differ from them.