This simple plugin will remove the link "Disable my messenger" from all members and will show an error if the user tries to disable via URL. If you want to restore the Messenger for existing users that has disabled it, run the following query in your SQL Toolbox:
update core_members set members_disable_pm = 0;
Append the DB Prefix to the table name (core_members) just in case your use any. Ex.: ibf_core_members.