How to: Delete a Proxy (SQL Server Management Studio)
When you delete a SQL Server Agent proxy account, make sure the proxy does not reference any active job steps. To check for any job steps that reference the proxy, right-click the proxy, click Properties, and click the References page. If you delete a proxy, you are given the option to reassign all job steps that use that proxy in the Delete Object dialog box.
After upgrading from SQL Server 2000, all user proxy accounts that existed before upgrading are changed to the temporary global proxy account UpgradedProxyAccount. The UpgradedProxyAccount is only granted access to those subsystems that were explicitly used, and does not have access to all subsystems after upgrading.
To delete a proxy account
In Object Explorer, expand a server.
Expand SQL Server Agent, and expand Proxies.
Expand the subsystem of the proxy account you want to delete, right-click the proxy, and click Delete.
In the General page of the Delete Object dialog, you can check Reassign to to reassign the job steps that reference this proxy to another.