As a teacher of English, what strategies would you use to prevent the use of the mother tongue when you ask students to work in pairs?

I always announce rules of the Spoken English class on Orientation day which includes:-

* No other language is allowed in the class. If anybody try to speak other language he'll get the only answer; SORRY! I don't understand.

Students are very comfortable to talk to their friends. When a student is encouraged to talk in English and not keep on correcting him/her all the time, he definitely gains confidence to talk and express himself/herself in English.

I think it is better not to let students know that you speak their language .If you were living in the same area , you announce in the class constitution that the language used is the target language.

The point is do we have to really avoid using L1 while teaching L2? What is the problem in using it to give instruction as long as the students strive to use L2 to do the group task, which is the important thing? I think over using that we need to talk about. Over using might make them develop the habit of depending on L1 to learn L2.   

base the class on values which are rewaradable only through usage of english

Thank you for the answers Anayat and Ahmed. I am sure the strategies you have suggested are useful to prevent students from talking to the instructor in the mother tongue. Now, how can we prevent them from using the mother tongue when they talk to each other? (pair work)