How to disable iMessage on MacBook Air only? If I untick 'Enable iMessage' in prefs on my MacBook Air, will that stop messages going to it? I only want them on my iPhone.

If I do the 'Delete account' thing as advised in one video, I am worried iMessage might not work on my iPhone either.
You can do either. 

Disabling your Apple ID in iMessage will allow you to quickly re-enable it if you choose to use it on your Mac at a later time. 

Deleting your iMessage/Apple ID account from iMessage preferences on your Mac will not affect iMessage on your iPhone. The only thing that it will affect is your ability to send and receive messages, either iMessages or Text Messages, from your Mac.