Shaykh Uthaymeen (may Allaah have mercy on him) said,” Teat them [these Shia] as they treat you. If they greet you with salaam, then return their salaams.
It is not good to expel them from the mosque, rather some of them may be from among the common folk who do not know anything and have been misled by their scholars. If you are clever and call them in ways that are better, you may be able to influence them.
Now if you oppose them and say “Do not prostrate on pieces of paper, do not prostrate on stones” and the like, if the matter may be settled there and they will give up these things, that is good. But (the problem is that) they will persist, and the enmity and hostility between you will increase.
What I think you must do is advise them first, especially the ordinary people.
Advising does not mean attacking their madhhab and false religion. No, advising means explaining the truth to them and teaching them the Sunnah. Then after that, if you explain the Sunnah to them, I am absolutely certain that if they have real faith, they will come back to the Sunnah and give up their falsehood. If that happens, then that is better.If it does not happen, then you should treat them as they treat you.
As for expelling them from the mosque, you [a simple attendee of the mosque] have no right to do that [only the administration of the mosque should expel them after repeatedly advising them with evidences and good manners if they hold that is best and safer for the communities deen].”
Ref: Liqaa’aat al-Baab il-Maftooh (no. 80, question no. 4).
Also Shaykh Uthaymeen discussed when a person can be expelled from the masjid here:
Shaykh Uthaymeen said, “Those who come to the masjids to innovate in it an innovation, they are to be prevented from it except if fitnah (tribulation) is feared from doing so.
Posted by Abdul Kareem Ibn Ozzie