If you no longer want to use your MyMonero wallet, there is no specific deactivation process.
You would need to transfer any funds that you have out of your wallet. And once your wallet is empty, you can simply discontinue use of the wallet.
The MyMonero wallet service does not hold any of your personal information, and after some time has passed, your wallet will become inactive automatically.
It will remain inactive as long as you don't use it.
If you ever wish to use the wallet again, you can easily re-access it by using the mnemonic seed phrase or private wallet keys to log in.
What are your reasons for wanting to deactivate your MyMonero wallet?
Be sure to check out our web wallet, desktop wallet, Android iOS wallet applications.
These are all extremely user-friendly.