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.
> While serving the warrant and encouraging a witness to attend parliament "the Serjeant or his appointee may call on the full assistance of the civil authorities, including the police."
From the Wikipedia article.
So the Facebook employee was in fact under compulsion.
