They could decline his offer, but they'd have to deal with a massive devaluation of their company on the stock market if they did. Their shareholders might just start dropping their stocks after the recent increase in value and Elon could just buy the whole outfit on the cheap.
Edit: I forgot to mention it, but Twitter could "fight back" by dumping stocks themselves and lessen the value of Elon's shares. Of course, that would screw Twitter over which is a win in my book.