Nobody can force you to accept a letter or package, even it comes via certified mail. You can arrange not to be home when you're expecting to receive something, or simply refuse to sign for it when the delivery arrives. However, that may not protect you from negative consequences that a letter may have been intended to warn you about.
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Rejecting the Mail
You can refuse certified mail by not answering the door, which ultimately will cause the post office to return it to the sender. In addition, certified mail may be refused at delivery. You won't be given the mail itself before signing for it and accepting delivery, but you are entitled to be told the sender's name and address before making the decision. The delivery person will note that delivery was refused, rather than simply being undeliverable. Unlike some other types of mail, you can't return it once it's in your hands -- signing for the mail indicates proof of delivery.
Rejecting certified mail doesn't make a negative situation go away. The willful failure to accept certified mail likely won't stop the foreclosure process, for example -- the noted refusal to accept the document is enough to indicate a good-faith attempt to deliver the needed notice. Others may resort to serving you key legal documents in person and then demanding you pay for the expense associated with that as part of legal proceedings.