Soccer prohibits 'violent conduct' without qualification, and there are a number of occurrences of players being sent off for violent conduct against pitch invaders. Sometimes this appears harsh, and on other occasions the reactions by the players are so disproportionate that the criminal law should probably get involved.
My view is that it would only be justifiable to use direct force, above the level of restraining/grappling, if there was a real reason to believe that the invader was going to be violent. This thread's incident involves throwing the invader to the floor, which was clearly not necessary, and appears to have been deliberate. I don't believe that's appropriate - that someone is invading an area they should not does not remove the duty of others to act in a proportionate manner.