Place a piece of broken clay over the hole before filling the pot with soil. The clay blocks the hole so soil can't escape, but moisture can still work around the clay piece and drain from the pot.
Line the bottom of the pot with a piece of wire or plastic screen material. The material holds the soil but doesn't inhibit water movement. Screen material works especially well in pots with multiple holes requiring plugging.
Line the pot with a coffee filter before adding soil and planting. The coffee filter works the same as the screen material, but it breaks down over time so the holes aren't permanently blocked.
Insert a cork into the drainage hole if you need a watertight seal, such as for a container water garden. Seal around the cork with plumber's putty. The cork and putty make a watertight seal but are easily removable if you need the pot for traditional planting later.