Latex - based
Remove the lid and let the contents air dry in your garage or basement. Make sure the open can is out of the reach of children or little critters. In a matter of days, the liquid paint will harden. When this happens, the can, with the lid off, can be placed at the curb.
To speed the drying of oil-based paint, you can add cat box filler, shredded newspaper or any other material that will absorb the liquid. The can with the lid off can then be placed at the curb.
You can also buy a “waste paint hardener” at home improvement stores.