I had this same problem on my clio. It seems that sometimes they screw them on, sometimes they rivet them on, sometimes they stick them on etc. Mine were riveted on like yours, I tried for ages to pull the plate off but i was worried that if i forced it the rivet would have just pulled a huge hole in the bumper.
In the end I went to my local renault garage and they took it off and screwed my new one on for me free of cost, only took about 10 mins. I think they simply took a drill bit to the rivet head destroying it and then let it simply fall out behind the bumper.