There are several methods that may work, you'll
just have to try a couple and see what works best
for you. Alot will depend on the type of adhesive
that was used and how it was applied.
Here are three ideas- each involve scaping-and elbow grease.
1. Heat- such as a blow dryer or paint gun to soften
the adhesive. Careful not to start a fire.
2. A solvent- depending on type of adhesive, your
local paint store should be able to help.
3. Dry Scrape- This is the hard labor.
Don't worry about getting it perfect, you will
have to rent a floor sander starting with coarse
paper and working to fine. The coarse paper will
remove the bits and pieces and the varnish from the wood.