If you knew someone was going to try krokodil, would you stop them?

I'd likely try to talk them out of it, and certainly wouldn't support them during their extended suicide.

Would you support a law to stop them?

I support laws to make it easy to avoid, and support for the induced coma while getting clean. I wouldn't incarcerate anyone for their choice of self-destruction. I'd probably support charges of attempted murder for giving it to someone without explicit written consent.