Their roots are pretty shallow; I'd think the best way would be to simply dig them out. You could use a weed killer if you really wanted to, but there are disadvantages. Round-up type chems have low toxicity and photo-degrade, but they work pretty slowly on succulent plants. Many other herbicides can leave residue in the soil for a long time...I'd be darned careful with them. As I said first-- I'd just get a potato fork & dig out the shallow rhizomes.

Over water when it is cold. It won't cost you anything except the water.