An “unannounced demonstration” by Nazis ends when upstanding citizens push past police, steal a Nazi flag, and set it on fire. The Nazis leave, and the resisters are not arrested.
These people were just going about their day, and it was important to enough people to stop and get directly involved. They took on risks of bodily harm and arrest. I’m willing to bet that none of them were “prepared” for the confrontation.
Uhauls are also fairly standardized, so one could put a padlock just big enough to fit but not big enough to cut off easily.