user list privacy

Hi there,

I found one issue related to privacy and spam.

While send new message,
What if user just type A or any other letter in TO field than it will list all the user data with that letter.

Also the user can span message to another users. How can I prevent them.

Is there any way to use this just for communication of Customer to Admin Or Vendor.
Customer can not able to message other customer. with using capabilities option?

Thanks in advance!