Technically, you can avoid certified mail forever, meaning that no one is going to force you to sign for it. But, you cannot avoid the creditor or the Court forever. If you fail or refuse to pick up the certified letter, the civil rules allow the creditor to send you the Summons and Complaint by regular mail to your house. Presuming the creditor serves you correctly, it's effective when it hits your mailbox and you need not sign anything. You should not presume that the creditor will simply give up because you didn't pick up the certified mail letter.
If you are indeed a named defendant in a lawsuit, I suggest you contact an attorney.