Fail to Obey

Fail to Obey
If you park your car in the wrong place,a traffic policeman will soon find it,most of time our will receive a ticket,it's bad,right?But the key point is you cannot fail to obey it,because we are all lawful citizen..But sometimes you have to obey something or someone and it's none of lay.For example,your friend want you to take part in a party,but you really don't want to go,how do you react? lie to them that you have no time,or obey them and say yes,because of it's easy to say.
Are you a polite people?Did you ever in this situation that when you are busy,but a person disturb you.How do you react?Say no to them,or cannot fail to obey?