Hey JW,
Is the account churning due to the CSM? We've all dealt with inevitable churn at times whether it be a change in the client's business model, weren't sold the right solution, company restructuring etc. At this point I would prioritize solving what the reason for churn is and gameplan with the CSM.
Now if the reason for churn is based on a bad CS experience like lack of follow through, radio silence etc then I would work on a performance plan with the CSM and have regular check ins as this could drip down to other accounts in their book.I agree with Craig and Shari on the offboarding. Even if it's time to move on from your product, you could still leave them with a great feeling which can lead to referrals, advocates bringing you to a new org, etc. From personal experience, there's always a chance they can come back some point down the road.