If it’s not a good fit, you should still send an email or letter explaining your rejection. It doesn’t have to be done in a crude or unprofessional manner. Instead, do these three things:
Thank them for their time.
Explain why you are rejecting the opportunity — be as detailed or as vague as you’d prefer.
Wish them luck in the future.
By remaining professional and timely in your rejection letter, you avoid burning a bridge that you might want to utilize later in your work life.