Clearing a MOS

Hello All

Historically we haven't altered the Mode of Sale while selling tickets.  As part of our desire to use Tess more effectively as an analysis tool we now require our front end staff to change the mode of sale according to how customers are booking i.e. walk-up, phone call, mail etc,  front end staff have never changed the MOS they are a bit hard-wired not to change it and are finding it difficult and frustrating. 

My thoughts are that I would like the MOS to clear after each transaction but I cant see a way of doing it.  Can this be done?

TIA

Alec