IT'S OK TO PUNCH NAZIS
The concept of libertarianism is deeply-rooted in broad opposition to various forms of authoritarianism. We, as Libertarians, hold the belief that any person who forcefully lays their hand on another without consent, in an effort to coerce or govern them is a tyrant, and therefore an enemy to the essential unalienable human right of liberty. Once hands have been laid, defensive force is not only justified, but absolutely necessary in preserving freedom and natural autonomy.
The authority of any government is derived from the fact that individual non-compliance will be met with official force; this implies that governments are inherently violent, since all actions are backed by brute physical force, or, at the very least, the coercive threat of force.
Since government is inherently violent, then, naturally, so is anyone who seeks to control or "become" government--at least to the extent of achieving their desired outcome beyond coronation. It would then not be a giant leap to conclude that most voters are violent, that most lobbyists who seek influence within the state are violent, or that most political donors are violent. All of these parties conspire together to act against the interests of others, through the time-honored traditional application of force, and, as with any conspiracy, the conspirators do not need to be successful in their endeavors to be culpable; they simply need to take proactive steps towards achieving their desired outcome, combined with the reasonable ability to follow through.
One individual is not likely to be able to construct a massive barrier that restricts the free movement of migrants, which would likely result in countless casualties of courageous people trying to exercise freedom of movement, along with immeasurable economic harm spanning international borders. However, when a large group of individuals band together, donning red hats and encouraging a xenophobic tyrant to lay the groundwork for these travesties, they are all liable for the violence that is inflicted. It should go without saying that individuals who participate in groups that have a stated purpose of elevating their own self-interests by oppressing others, and take any step toward achieving those objectives, are acting aggressively.
When someone is acting aggressively against another, self-defense is appropriate, according to libertarian principles. The amount of defensive force used should be proportional to the threat imposed. Proportionality should consider not just the immediate consequences, but the long-term consequences as well.
A long-term consequence of being subverted, oppressed, and/or effectively enslaved by a government is far more significant than a superficial injury that will leave no permanent lasting damage. Defensive force can be used on behalf of others as well. You don't need to be the specified target of the aggressor to be justified in intervening on behalf of the intended victim. As capable, able-bodied, righteous human beings, we may exercise our right to intervene whenever possible, in defense of liberty and justice for all--and our right to preserve our collective well-being shall not be infringed.