There are three possibilities:
1) you didn't actually uninstall it (e.g. you just removed it from the desktop rather than uninstalling the app)
2) this is a different "memory booster", either built-in or which you installed (perhaps as part of something else)
3) these are not messages from a memory booster app but ads for that app being pushed by something else
The first thing is to check that it's really not installed. Go into your Settings > Apps and make sure it's not there. If it is, kill it with fire (all "memory booster" apps are scamware, and have no place on any android device).