As libertarians, we are against the use of aggressive violence. That is the essence of the Non-Aggression Principle, which states that it is wrong to use violence or the threat of violence against peaceful individuals. These peaceful people who ought to live free from the harmful coercion that the Non-Aggression Principle forbids are those who are acting within their rights.
The Non-Aggression Principle does not apply to people who use aggressive violence. As a result, it would be permissible to use some degree of violence as a defensive measure.
Some people, including some libertarians, believe that the state has the authority to restrict immigration on the grounds that the immigrants may end up on some sort of welfare. There are several things wrong about this position from a libertarian standpoint, including that the state has no actual authority to do anything and that it is wrong to convict these potential immigrants of any “pre-crime” (i.e. it begs the question), but we will suspend these objections for the time being. There is an aggression that is committed during the process of taxing citizens to provide welfare for immigrants (or anyone): taxation is theft.
The criminal in this situation is the state. The immigrant who may at some point receive the welfare is the unwitting recipient of stolen goods (unwitting because most people do not agree with libertarians that taxation is theft). Yet the libertarian who favors the restriction of immigration directs his advocacy of violence against the immigrant and not the criminal state apparatus.
This is a contradiction of the Non-Aggression Principle.
So given a situation where a libertarian would advocate the use of violence, why would he advocate it against the people who are not the aggressors? Why advocate the use of force against people who have not yet committed a crime? If you are going to advocate violence, shouldn’t it be against those who are doing the explicit harm?
If someone were to take the anti-immigration position because of the welfare issue, would it not also make sense for them to take a position that it is acceptable to initiate violence against anyone who receives welfare? But I am not aware of any that do despite that it would be a more consistent position.
This contradiction of the Non-Aggression Principle along with the other objections mentioned above provide overwhelming evidence that it is incorrect for a libertarian to support the restriction of immigration by the state. A consistent libertarian should never argue for the state to do anything.