Remove shaper should do exactly that. If you are still experiencing issues, double check your modem by connecting directly and performing a speed test. I'v ehad this issue myself, before, and learned that there was an issue with my DSL2 link that was the actual limiting factor. A call to the ISP resolved it once a tech came and found a faulty bridge that had allowed water to seep into it.
If you still experience issues, double check the limiters page, and make sure that all floating rules are removed (which should have happened then you clicked remove shaper).