I think they carry a ceremonial mace, heavily ornamented with gold reliefs:

https://en.wikipedia.org/wiki/Serjeant-at-arms#/media/File:M...

I'd definitely not want to be bonked on the head with that. Fortunately, as the article you link to says, they can call the police to do the dirty job. "Fortunately", as in, the combined threat from the mace and the cops should suffice to convince most people.