Comic Strips

In anarchocommunism, those that use violence to enforce it wouldn't have monopoly on violence, they would be group of volunteers from the community. Those volunteers could act under a mandate given to them by the community, or they could organize spontaneously and take justice into their own hands. The community can retroactively endorse, tolerate, or condemn their actions. If their actions are condemned by the community their mandate can be revoked if they had one, and they can be held accountable nonviolently or, if necessary, opposed violently to prevent them from causing further harm.

Defending an anarchocommunist society against capitalism would likely happen through guerilla warfare, like how the Taliban successfully defended itself against NATO. The guerillas would also naturally be volunteers without any monopoly on violence, and their actions could either be under mandate from some revolutionary commune or spontaneous. Offensively, guerillas win by bleeding the logistics of their opponent dry, leading to the mass surrender of underpaid and undersupplied soldiers, allowing the guerilla to storm the halls of power with minimal resistance. Less dramatically, anarchocommunists can win political victories through terrorism (like the British suffragettes), riots, strikes, and more.

That said, violence is a measure of last resort. Most of the time in an anarchist society it's enough to tell someone they are doing a bad thing and help them with unlearning it, to redesign the tools or systems through which they caused harm, to no longer give them the tools that allow them to harm others, to warn others of their history of harm an divest from them, or to be a barrier between them and the tools that allow them to harm others, or to threaten them.

So if you had a group of enforcers that abuses their capacity for violence, then firstly their mandate can be revoked. If that doesn't work, people can try to take their weapons. If that doesn't work, people can sanction them. If that doesn't work, people can follow them around and attack them if they try to use their violence to intimidate others. If that doesn't work, people can ask the help of other communities to help defeat them. If that doesn't work, people can burn their houses down. If that doesn't work, people can assassinate them.

If that doesn't work, people can go scorched earth: evacuate the path ahead of them and destroy whatever they need to survive until they starve to death. If that doesn't work, people can surrender and murder them when they let their guard down. If that doesn't work, people can engage in economic sabotage and propaganda for revolution. If that doesn't work, people can make art and inspire a dream for a brighter future in later generations while mitigating what harm they can. If that doesn't work, well, they'll be long dead before they know that for certain.