It is explained when you go to your account and then the advanced tab:
Advanced
If you would like to disable your account, leave your current company, request your account's audit trail, or get assistance with changes to your account that you cannot perform here, please contact Mendix Support.
So contact Mendix support to disable your account.
Regards,
Ronald
Maybe you shouldn’t delete your account. Look at the following.
https://gettingstarted.mendixcloud.com/link/module/66/lecture/446