I have a online stock broker and I got an email from them saying they are banning me from trading can I sue?

I will not name the company but I have been with them since 2004 trading online and all of a sudden they email and said because I'm not in the U.S I can not trade stock anymore only sell the stocks I have. I'm an American and I wasn't in the U.S when I sign up with them I was working overseas for the U.S. Military. Now I decided to take a year off they notice I logged into my account from Thailand and they put this restriction on me that I can only sell my stock. I was just reinvesting my dividends but they stop that too. I'm an American citizen.