It is preferable to actually apply the rest of the paint first. If this is not possible, then surplus paint should be disposed of safely. It should not be poured into drains, watercourses or placed with household waste as is. If small amounts of water-based paint need clearing up then throw some absorbent material (most pet litter is suitable) over the paint to soak it up then sweep it up and throw into the dustbin. For larger amounts and all solvent-based product disposal, speak to your local authority who will advice on the best course of action.
If you have left over paint in good condition consider donating it to charity or community groups, again your local authority can provide further detail on the schemes operating in your area.