If you need to securely close a really heavy bag, one option is to use rebar wire ties.
You can buy a pack of 100 rebar wire tires at the Home Depot.
Grip-Rite 67/100 ft. 16-Gauge Rebar Tie Wire (100-Piece/Bag)
To secure, make a neck out of the plastic bag, fold the rebar wire tie around the neck, insert the screw end of a hook into a drill, insert the hook end of the hook into both loops of the wire tie, pull the trigger on the drill to twist the wire until it secures (chokes) the neck of the plastic bag.