Pry bars, long drill bits to break up the bigger chunks, followed by a a good " sucking out" with a shop vac. Get to the bottom, clean off the perimeter, and the new post should slip right in. Some planing on the new post may be needed - standard size posts can vary in dimensions.
Check the size of the shaft. Hot rolled steel tends to run oversize. Also, you might try to intentionally re sharpen the drill to a non-standard configuration, ie. with unequal angles or unequal cutting lips. This will tend to make the drill drill oversize.