Hi,
There is no native way to disable the IM for user but you could check these discussions that happened earlier where you see the registry mentioned , or if you want to do it for group of people then there is third party solutions.
https://social.technet.microsoft.com/Forums/office/en-US/4b7eb43c-84c9-452f-bb04-c7ede867a2ce/lync-2010-is-there-a-way-of-disabling-im-communications-between-departments-or-ad-groups?forum=ocssecurity
http://corporateitsystems.blogspot.in/2014/03/disable-chat-in-lync.html
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Linus || Please mark posts as answers/helpful if it answers your question.
