Disabling comments on users own nodes

Hi!

I think that 'disable comments on users own nodes' option would be very helpful.
Imagine that user offers his service on the site and registered clients comment his service. But when the clients are not satisfied with users service and post a comment about it, offering client may start unwanted argument using comments on his own node. I would like to disallow this user to do that and block comments on his own content. Also this user may add some free info to his service for which he should pay when he bought a restricted service ad node.

Is it possible ?

what code should I add to get such functionality ?

Thanks and I really appreciate every helpful post.