How can I permanently remove iMessages from my Mac desktop?
How can I remove iMessages from my desktop Mac?
iMac
I think El Capitan has SIP (System Intregaty Protection), which prevents users from deleting or modifying standard macOS components, which will often result in costly customer support calls.
My suggestion. Do not login to messages, drag it off the Dock, and forget it exists.
If it truly annoys you, that you must have it off your Mac, then Google “Mac disable SIP”