Possibly one of the most annoying traits a coworker may ever have, checking your table without permission is one of the typical traits of the nosy colleague.It amazes me up to now why some people just can't help themselves from peeking at others' work and personal belongings. It doesn't even start inside the office, in the first place. Even in university, there will always be nosy people who like to check personal belongings, as if you are hiding some weapons of mass destruction or anything like that. A lot of them think you guys are close enough that they already gained the right to do their nasty deeds. Good thing I am not experiencing much of that now.
If in the university people check bags like they are some kind of security personnel, in the office, some coworkers like to harass office tables. Worse, a lot of them seem to like to decorate your tables for fun. Why can't they just bother to decorate their own tables? If they are unfortunate enough not to have their own place (which means they just probably do menial tasks *boohoo*), then I suggest you give them one of these molded tables. It seems unfitting inside an office, but you shouldn't bother giving them anything expensive. They're just going to decorate it, anyway.
I am sure that you will agree with me that it isn't about you hiding any dangerous or anything that will put you at a disadvantage. It is about respecting what you deem to be your own space that nobody can just step on without your consent. So when a coworker checks out your table, then go tell him/her you don't like it. That's the only way he/she will stop.