I am pretty sure you are entitled to cancel the lease if your landlord is aware of the problem and is not doing anything about it. You might have to prove that the landlord knew about the problem all along, and that might require either witnesses to attest that you did talk to him, or a written document, such as a dated letter informing the landlord about the problem. Your best bet is to check your local tenant-landlord laws to see what exactly you are entitled to and what do you need to make your case.
You can usually find lanlord and tenant law information on your state's official website, the State Department of Consumer Protection website or the State Attorney General's website. Go to the state official website ans search for consumer protection, or landlord /tenant law.
