The same procedure that is used to ban non-Muslims. If they want to lie to get in, the Muslims can't do anything. Historically speaking, a few non-Muslims have gotten into Makkah by lying - and the authorities are blameless in those cases, too. We do what we can based on the haqq and leave the rest to Allah .
And again, most of the ulama would not support a general ban on the Shi'a, but they would propose a ban on those that attack the sahaba and insult the wives of Rasoolullah or those that believe in the divinity of imamah or the tahreef of the Qur'an and other such nonsense. Most lay Shi'a don't know the details of these corrupt beliefs, unlike the Shi'a rabbis.