An individual entitled to be paid an annuity may, for personal reasons, decline to accept all or any part of the annuity. However, a principal may not waive the portion of his/her annuity authorized to be paid to a former spouse under Sec. 19.7 or Sec. 19.9 or to a beneficiary under Sec. 19.6. An annuity waiver shall be in writing and sent to the Department (PER/ER/RET). A waiver may be revoked in writing at any time. Payment of the annuity waived may not be made for the period during which the waiver was in effect.