In order of escalation...
1. Inform them they violated a term of their lease and the smoking must end immediately.
2. Inform them they may be responsible for any smoking remediation that needs to be done on their move-out, such as carpet and curtain cleaning, painting, etc if there is a smoking odor or residue.
3. Charge them for smoking remediation (#2 cost made real). Ideally, you get them to pay you directly for the cost. You could charge their security deposit, but it likely wipes it out leaving you with no security at all. You could re-charge them to refill the security deposit.
4. End the lease or evict them. This is the sledgehammer to be used only if this is a major issue for you and you are willing to take the cash flow and renovation impact now.
I would do #1 and #2. Discuss the issue with the tenants before sending them a letter. #3 Depends on their reaction in the discussion. A promise to cease and desist that is kept would end my escalation. Otherwise, I might do #3, but probably only on move-out. I would only end the lease or evict if I thought I caught the smoking early; otherwise, the damage is done.