If you wish to pause student usage of Conversations, you can temporarily hide the tool through using the Sakai Site Info tool. 

Once you are in Site Info, select Tool Order from the tab menu at the top. 

Within Tool Order, find  Conversations, and select the gear icon to the right to go to the Conversations settings menu. Select Lock Access to this Tool from the drop-down menu; a padlock icon should appear next to the gear menu.

When you are ready for students to resume use of Conversations, return to the tool settings menu, and select Enable Access to this Tool. 

Note: Do not select the option Make Tool Invisible to Students. Invisible tools are hidden from students in the Sakai left-hand menu, but they can still be accessed via direct link.