How would an agent rescind the active POA?

The principal has to revoke the POA.

Read the POA carefully. Some provide that an agent may resign with notice to the principal and a successor agent. If this one does not, talk to the principal. No one can force someone to be their agent but if you signed agreeing to be the agent, you will need the help of the principal or, if the principal is unable or unwilling to let you resign, an elder law attorney. (www.naela.org)

Only the principal can revoke a POA.