To my knowledge, it isn't. There is a date of ownership where the dividend eligibility is determined and then it is paid out a few weeks later. I would imagine those payouts are binding but a planned dividend for June could easily be rescinded without legal penalty. However it is priced into the value of the stock. Slashing dividends will tank the stock price.
Eliminating a planned future dividend by itself would have no effect on the stock price. What would affect the stock price is the management explanation that comes along with cancelling the dividend. If they say that profits will drop even more than analysts had expected, then that would tend to cause a share price decline as it would be new information to the market.
Stock prices usually drop after a dividend is payed out because each share now represents a claim on fewer assets.
