Yes. Otherwise we're accepting "OpenAI wants to do this so we should quietly get out of the way".
If ChatGPT has "PhD-level intelligence" [1] then identifying people using ChatGPT for therapy should be straightforward, more so users with explicit suicidal intentions.
As for what to do, here's a simple suggestion: make it a three-strikes system. "We detected you're using ChatGPT for therapy - this is not allowed by our ToS as we're not capable of helping you. We kindly ask you to look for support within your community, as we may otherwise have to suspend your account. This chat will now stop."