Hello Gaurav. For the problem with sending messages to students, it might be helpful to enable debugging for a brief period, and then try to reproduce this issue. I am wondering if any debug errors will appear that might be helpful in troubleshooting this. Perhaps there is some issue with the email server or something else, but lets see if any helpful debug messages come of this. Also, you say that no confirmation message displays... what is displayed as a result of the message not working? Also, what functionality are you using to send a message to students... is it a forums/news activity, or are you using the "send a message" option from the course participants page?
For closing a course, one option you might consider is to edit course settings, and set the visible option to "hide". This makes it so that students cannot enter the course anymore, but teachers/site-admins still can.
Jamie