How can I remove a bottom plate that's been glued to the floor?

In this case, an "Undercut" saw or an oscillating saw with a blade that will cut through metal (spikes/nails) is the way to cut it away and leave a clear flat surface. The other option is to use brute strength but be aware that the subflooring will be damaged. The subfloor will normally be plywood and any damage done to the top laminations can be leveled and cleaned up by using self-leveling floor compound and trowling it out as you pour it. Be careful not to use to much self-leveling compound as it is possible to build a hump onto your floor.