Any system that SIM is monitoring is a managed system. In your list of managed systems, one or more of them are associated to your network devices that are getting these logon attempts. They either refer to secondary IP addresses or web farm IPs or load balanced IPs...something. So when SIM tries to manage these IPs, it tries to log into them with the credentials that it has stored, in the global credentials settings. If you remove these global credentials but copy them to the existing managed systems, it won't solve anything. You have to find the managed IP entry in SIM that it is using to contact these appliances and manually remove these credentials.
You could also choose not to copy the credentials and that will solve your problem immediately. It may also create a new problem where previously managed system can no longer be managed because they don't have credentials matched up to them. In that case you would have to manually set up credentials on those systems.
