If someone or some company sends you certified mail and you're alerted in your mail box that you need to sign for some certified mail and you fail to sign for the certified mail or pickup the certified mail then the certified mail will eventually be sent back to the sender of the mail.
You cannot be forced to sign for certified mail nor can the post office force you to or the company cannot force you or anyone else to sign for certified.
No one is required to sign for certified mail so you can legally refuse to accept the certified mail if you want too however it will just be sent back to the sender and then later the company or sender may try to resend it again.
But it cost around $6.00 each time the certified mail is sent through the mail so they may not try to send the certified mail back to you often.