To answer your questions about lashon hara and to publicly embarrass someone in public. This is a big problem. You can't see someone at an event and tell them in public "get out, you're not welcomed here" for various reasons. To humiliate someone in public is akin to murder. Unless someone is a well known rasha, I don't foresee any shul getting to that point. The rabbi would be extremely concerned about the question of humiliating the person or other outcomes.
I can only think that if there is a shul event and some messianics come in and start preaching, they will be told to get out. If it's someone who the rabbi and others perceive to be a frum Jew, they may not want to kick the person out publicly. In this case, it should be done tactfully. Expose the person and probably have it be done in private. I have heard instances of a persons actually getting kicked out from a shul but they get a letter from the president or get called by phone or into a private meeting.
To explain the implications, there is a Gemara that says: “A person should cast himself into a furnace of fire rather than publicly humiliate his fellow.” and in the Torah, one of the mitzvot says that while you can rebuke a fellow "ou shall not bear upon him a sin" meaning that when you rebuke someone and the person deserved the rebuke, at the same time one must not do so in a way that will humiliate him. But then you get into the technicalities of "how many people present make it public humiliation".
To speak lashon hara, motzi shem ra and rechilut is to also kill a person, and even more than one person is harmed by it. It is the person speaking it, the people hearing it and the person it is spoken against. Very big problem and the easiest one to fall for. The Chofetz Chaim in his Shimras Haloshon gave the specifics on all situations in which lashon hara should not be said or avoided. He points out the problems, implications of this serious problem.
However, the Chofetz Chaim DID specify when lashon hara is permitted! Yes, in some instances it is permitted and that is when a person is evil and is causing great harm to Klal Yisrael. If someone is an immediate danger to the community, then it is permitted to speak lashon hara on them. There are some specifics, however, that one must try to refute the person. If the person does not change then the community must be warned. I believe, however, the last prohibition is that we are not supposed to enjoy it. It should not be pleasurable to speak ill of someone, it should be out of a necessity to warn to community.
Even though you are right about this Jacob character, and since he is an enemy of Israel so it is technically permitted for you to expose him without having to worry about lashon hara.....it does not mean that other people will hear you out. With the laws of lashon hara hanging over our heads, many people just completely close their ears and will refuse to listen to something negative about a fellow Jew, even if it's the truth. That is why it's important to be tactical about it.