When a dispenser tries to place a shulker box, make a fire with flint and steal or use bonemeal on a plant, but the block in front of it is incompatible to that action (everything except air for shulker box and flint and steel and anything except growable plants for bonemeal), it will do nothing instead of dropping it, like it does with water, lava and empty buckets, boats, minecarts, armor, weapons, tools and items that don't have a special action for dispensers.