about people leaving stuff on desks to “save” or “claim” a seat, you are a big part of the problem. Understood, most rational people want to avoid conflict. But when you say nothing, and do not challenge or report this, you are enabling the practice and you are justifying the flawed policy of no assigned seats.
Grow a pair and sit in that seat. Report the offenders to GRE, copy their bosses and your boss. Every single day if necessary.
If conflict arises, make sure everyone knows that you do not feel safe. Call the police if necessary.