Paint hardener is not the only safe way to dispose of paint. Other items that help include cat litter, sawdust and cardboard. The type of paint dictates the best way to dispose of it.
Latex paint needs to be dried out before disposing of it. One way to do this is by pouring it onto cardboard if it is only a small amount. Pour it on the cardboard and paint it around the surface, waiting until it dries. The cardboard is then recycled or thrown away. Another option is to add kitty litter to the latex paint can. There needs to be equal parts kitty litter and paint in the can. Stir it until it becomes a thick consistency. Let it sit and dry, then dump the mixture into a trash can.
Oil-based paint is a hazardous household waste, therefore it should be brought in the can to a local hazardous waste vendor. Disposing of it at home is not recommended. With spray paint, spray the rest of the contents onto cardboard, then dispose of the can in the recycling bin.