Is it better to stop performing Ruqyah for those who seek it after resorting to sorcerers and graves, and then return to their polytheism after being cured?

There is no harm in performing ruqyah for innovators (ahl al-bid'ah). In fact, it is advisable to continue counseling them, reminding them, and teaching them, as this represents an investment in the path of inviting to Allah. The proof for the permissibility of performing ruqyah for them is the consensus of scholars on the permissibility of a Muslim performing ruqyah for a disbeliever, as mentioned in the hadith of Abu Sa'id al-Khudri regarding performing ruqyah for the stung person with Al-Fatiha. If performing ruqyah for a disbeliever is permissible, then performing ruqyah for an innovator is even more permissible.