You can either trash them or donate them to the Dumpster Dive. Go to your inventory page under the "My Gaia" tab. Click on the item you want to get rid of. On the bottom, you'll see buttons that perform functions (add to aquarium, trash, donate, close) Click on "trash" if you want to permanently delete the item. Click on "Donate" to donate it to the Dumpster Dive so someone else can have it.