An employment lawyer would need to evaluate the contract and your working situation to be able to give you a careful evaluation. If you are working more than 40 hours, then you may be entitled to overtime pay. I suggest that you consult in person with an employment lawyer in your area. Good luck.
Employment contracts are binding, provided they otherwise meet the legal requirements of a contract, includng an offer, acceptance, consideration, not signed under duress or fraud, etc.
If your contract said you will only work 30 hours a week and you are working 40 or more, then depending on what the contract says, you may be entitled to additional pay. Also, contracts normally state the conditions under which they can be terminated. Even though you have a contract, it may contain an at-will clause. This means you can be fired for any reason, or no reason at all, at any time, with or without notice. Under this concept you could get fired for not working more than 30 hours.
It is hard to provide accurate advice without seeing the actual contract. You should consider speaking with a labor and employment attorney before you just refuse to work the extra hours. An attorney can also advise you on steps to take to improve your chances of collecting unemployment compensation in the event you are terminated.