Just say Iie or No
In Japanese, the direct expression for no is iie. However, most Japanese people know the English word no and are more likely to respect a direct refusal. In Japanese culture, refusal is often done out of courtesy and Japanese people will ask a few more times unless your refuse directly.
If you are looking for a more polite refusal, you can try Kekko desu (it's fine) or sore wa chotto (that's a little bit too much).