I propose to put the lich's phylactery in a bag of holding. Then, stick sword through the bag, thus destroying it. According to the DMG, all items in the bag are lost forever. No mention of them being another plane...just lost forever. Now, the lich is a level 20 wizard. Still, I am trying to figure out why this won't work. Seems way too simple. Also, the phylactery is considered an artifact.