If you are still in the castle, you can fill a chest with everything you do not need, and toss it out the front gate (hold the shift key down to move the screen focus, to throw it farther).
If you are in Iona's dungeon, simply place a barrel in a convenient location and fill it with stuff to be picked up later (place a flag on your map, so you can find it again). The lower levels of Iona's dungeon are optional, so you can leave them until you get your teleporter stones back.
Once you leave Iona's dungeon, you will get a quest to rescue Zandalor. Do so and let him teleport you somewhere to get the teleporter stones back.