1. Steam cleaner with stiff wire brush attachment. This works but strips off the finish from the wood; so only recommended if you are planning on deep sanding the floor afterwards.
2. The better way is to sponge with white spirit and then use a paint scraper. The white spirit softens the glue so that the paint scraper takes it off easily. This has the advantage of preserving the floor finish. I am using this with an old oak floor and it works nicely.