Delete the associated certificates & keys from your local machine - Mac
To get started use Command-Key / Spacebar (opens search)
type "keychain".
it will bring up keychain access app in list
click keychain access app
Select the 2nd option [Open Keychain Access]
You'll have to supply a password or whatever to get in
Look under Certificates tab
You'll find a developer certificate for the account.
Be very careful you don't delete the wrong one
You should be able to delete the associated developer certificates and it will remove the developer account -- the certificate that XCode uses to sign the apps with.
You can probably determine which one is which by the associated email address. ie There is one email address tied to each developer account. Good luck