The police could enforce the situation except that they tend to be on a different side to the Army.Lots of reports of off duty policemen being involved in the previous Red Shirt problems.Army hierarchy in the past were loyal to the Yellow shirts-probably not as solid now though.
However the reason it is all a problem now is that the King's health is not good.The Crown Prince is a divisive figure but ordinary Thais will still be swayed by their king.As well the Bangkok elite will have to be wary as there is still the lese majestere laws outlawing criticism of the King.
Thailand Protests And Instability - Business Insider
The Crown Prince has recently been appointed to the Defence Council and put in charge of the Royal Guard meaning he is definitely the favourite in the succession stakes.The problem for the establishment is that he is seen to be very friendly to mr Thaksin.I read this article in the economist on my flight to DFW which describes the situation-
The future of Thailand's elite: Helplessly hoping | The Economist