As far as I understand, you do not want users to have multiple accounts of your service in a device. I see two options here:
Manage if user has already added an account. When user navigates to Settings > Accounts > Add account and choose your service, you will check if there is any registered account on the device. If there is, reject the new authentication.
Disable
"myPackage.authenticator.AuthenticationService"
right after user adds her first account (may not be safe), so that system (Settings app) will not be able to locate your service when it is parsing for authentication providers.