In plesk there is a button to enable or disable webmail for a domain.
How do I manually turn webmail ON or OFF for just a few e-mail accounts under a domain?
So some should be able to login and some shouldn't.

Webmail is a domain by domain basis not user by user.

Is there a way to block some users using webmail?

You would have to script a solution...possibly php..but no...plesk doesn't have an option to disable it user by user....as enabling webmail for a domain is really just a matter of a dns A record for webmail.domain.com to the name based hosting ip# of the server.

Thanks for the answer!
I have no idea how to do this with php,
Anyone can get me started?

there is a thread at forum.plesk.com that asks the same question....as far as I know there has not been a good answer there either...

but anyway...I am not a php programmer..so I really can't help....sorry.