A. Latex Paint Disposal
Just search for one of the many recycling locations where you can drop off your leftover paint for free. Some of these recycling locations also offer free leftover paint to anyone and some strain and mix into boring colors. Nice.
Paint can be toxic and dangerous, please follow the paint-disposal procedures in your area.
Harden it. For latex paints, just pour in this product and wait. Then put the solid in your regular trash. (Check local laws, but this is usually permitted).
More expensive than #2: Mix cat litter into the latex paint. The wait time for drying is much longer than option 2 above.
I’d take that lump of goo and pour it out on old cardboard (with that on top of plastic) and it would dry faster.
B. Oil-Based (and solvent-based)Paint Disposal
Solvent- and oil-based paints are hazardous waste and must be disposed of according to your local laws. Usually, you need to find a recycle center for paints. Sorry ’bout that.