How to solder QFN-16 with a central pad component to proto-board?
This should be a non-issue. Since you are not using the center pad, you should not tin it with solder, nor add solder paste below it. Just do the same thing you would do if it didn't exist.

Use a soldering iron and some de-solder wick to clean the center pad as best you can, and reflow it again.

If it really is a problem, you could try drilling out the center of that dip adaptor. This would give any pooled up solder paste somewhere to go instead of making the chip float.