In an organisation, the employees are used to use chat applications
installed on their computers during normal and off working hours. The
managing director has requested the IT manager to restrict their
access to those application to save company?s time and to keep the
staff busy only with their official work.

From a professional management perspective, is there a need for
distributing memo or sending and email explaining the reason behind
this decision? If yes, what should be put on that memo or letter?

Surely, it's self-evident why the decision was implemented?

What do you think could be written?

Unfortunately our Internet usage records show that some members of
staff have been using the system for non work related Chat and
Browsing.

From now on, please avoid non work related Internet usage during
official working hours.

The jist is to give a hint of a threat, and the suggestion of an
amnesty for the culprits.

offcouse,It sholuld be explained why the decission was implemented.

Just mumble something about viruses as you walk away shaking your head.