I've recently deleted contact details of a friend. Will this stop them from sending me an iMessage on my Mac? My iMessage is set up with my email address, not a phone number.
In short, no. If you want someone to stop sending you iMessages, you have to explicitly block them.
To block someone, open the Messages app, Go to Preferences, Accounts, and click on the Blocked tab. Hit the plus sign to add someone to the list.
This only blocks iMessages and won't block normal text messages (if you have an iPhone and it's set up to do handoff to your Mac).