Zactly.
The interlopers know that:
- Most people will slink away
- A manager won't take the stand that chairs are 'owned' so what's left is an aribitration. Which they'll stand up to, ending in a waste of time and the prior people slinking away anyway.
So as someone else mentioned with the "sit in their lap" comment, if you don't want to slink away, you have to out-crazy them, and find THEIR trigger. Sitting in their lap, or very close to them, or being noisy, or messy, or stinky - these may work. Usually a family like this is led by one bold bully and the rest are pretty normal - evenually the uncomfortable normal ones will side with you against the bad seed leader.
So I would actually NOT focus on or come close to the apparent alpha dog, rather the rest. Be as passive agressive as possible, which is to say that you don't recognize their existence, yet make it very clear that every action (and a continuous, unrelenting wave of actions) is directed at them, with no end in sight.
I'd focus on personal space. If they took the chairs, sit right next to the chairs and in a way that intermingles their people with yours. Continually fold towels and bring more things into the space (towels, food, etc). Start a song like 99 bottles of beer on the wall, or a game like "I'm going on a picnic". And at any point they give up some space, quickly fill in the gap and expand your domain.
And have some fun with it. They took your space for their benefit, take it back in a way that is for yours.