They want to prevent entry those people into their country. And it will depend on what that person said, and what they intend to do in the US. If someone posted that Kirk brought it on himself then they may be allowed entry for a weekend stay, but they won't be able to get a long stay or work visa. But if someone posts a video of themselves celebrating Kirk's shooting - that is, celebrating and condoning the murder of an American citizen, and calling for others to be killed in a similar way like this ex-councillor hag - then they won't be able to set foot on US soil. That's best case, for them.