Hello Marc,
I suggest trying Oracal 631, which is a matte, removable adhesive vinyl perfect for walls. You can request a sample
here.
I've attached a pic of the 631 on my wall that I applied about 2 weeks ago just testing it out. The symbols are scraps from another graphic I cut. Comes right off but you have to get a good grab on it with a nail and just pull. You can actually see a slight bump in the top of the diamond where I've gotten a nail under and pulled.
MACtac makes a similar product as well.
Miraentu Interior makes a lot of cool wall vinyl designs made of the same type of removable vinyl.
AB