This should be solved asap, especially if it was going on for more than a year.
First, ask everyone if they took anything from anyone else, if 2 or more say yes, then just try to shrug it off, knowing you aren't the only one. If everyone says no or if you don't have enough patience/self-control, then proceed to step 2.
Take Zack's suggestion and try to keep it in a hard to reach spot or a locked spot. This can really help you in the long run, as everyone won't know what to do due to you not giving it to them. I mean sure, sometimes people have to share, but if it's going as long to the point where it's been more than a year, I would say it's going overboard. If that fails, and they still are able to get your things, then go to the final step with caution.
Fully admit that you are so loving tired of people literally stealing things from your room without your permission, and you cannot take it any longer. From here on out, you can do anything, just try to keep in mind, don't let anyone stop you until you are finished to fully get your point across.
The best advice I an give.