In California the buyers have the right to check out the property and change their mines within a time period that is specified in the Contract. You would need to check with your agent and find out if you have such a time period in your state.
I assume you are talking about declining an offer on a property that you are selling. You have the option to decline any offer, just as long as the reason doesn't violate the Fair Housing Act. However, unless you are planning on withdrawing your home from the market altogether, we would suggest that you make a counter offer at the Price & Terms that are more acceptable to you.
If you provide a little more info about your situation, we can give you a more targeted answer.
Best of Luck!