Do not ignore the invitation. Respond in a timely manner, and let him/her know that you will be unable to attend. Never lie, and never over explain. Providing an explanation is not necessary, so do not feel obligated to do so. Always thank the person for the invitation, and offer an apology for not being able to attend. Do not say anything that could lead the person to think that you would be interested in rescheduling, such as "Maybe next time," "Let me check my calendar and get back to you," or "I'll call you, and let you know." Keep it brief and simple. "I really appreciate the invitation, unfortunately, I will not be able to attend" will suffice. If the person invites you later on down the road, just repeat the same process. Most people will get the hint, and move on.