You can't delete administrators in Sophos Central Partner but you can turn off access in Sophos Partner Portal. See Sophos Partner Portal.
To turn off access, do as follows:
Sign in to https://partnerportal.sophos.com/.
Go to Partner Profile > Manage Users
Select the name of the administrator.
On Edit User, scroll down to User Access and Roles.
Change Central Partner Access to No.
If you also want to prevent access to Sophos Partner Portal, change Active to No.