Whoa, executing them might make them bigger heros in the eyes of their supporters aswell as make the government look like a bigger oppressor. Just ignore the shit out of them like a toddler throwing a tantrum while making sure that they don't hurt themselves or those near them.

Exactly last thing you want are monarchist martyrs.

That's how you ended up needing to arrest a bunch of morons who post all day about wanting to do exactly what they ended up attempting to do.

Instead what needs to happen is if execution is off the table, you very purposefully remind them, repeatedly, that there is nowhere to defect to -- you will not have quiet enjoyment of your Nazi coffeehouse, your Nazi bar, your Nazi... whatever... because the antifascists will not tolerate your behavior and invoke stand your ground the second you even hint at acting on your antisocial urges.

I'd suggest that a humane manner of execution does not exist because murder is fundamentally inhumane.