Based on the following information taken from the OPM website, it does not appear you are eligible to cancel your FEHB outside of an Open Season. Outside of an Open Season you can cancel coverage only in connection with certain events called qualifying life events (QLEs).
The major QLEs that permit change in enrollment include 1) A change in family status such as marriage, birth or adoption of a child, acquisition of a foster child, legal separation, divorce, death of a spouse or dependent; 2) A change in employment status such as you are reemployed after a break in service of more than 3 days; you return to pay status after your coverage terminated during leave without pay status or because you were in leave without pay status for more than 365 days; your pay increases enough for premiums to be withheld; you are restored to a civilian position after serving in the uniformed service; you change from a temporary appointment to an appointment that entitles you to a Government contribution; or you change to or from part-time career employment; 3) You or a family member lose FEHB or other coverage under another FEHB enrollment because the covering enrollment was terminated, canceled, or changed to Self Only; when enrolled in a prepaid health maintenance organization (HMO) and you or a covered family member move or change worksite outside of the HMO’s enrollment area; under another federally-sponsored health benefits program; under Medicaid or similar State-sponsored program for the needy; under CHAMPVA, TRICARE, or TRICARE-for-Life; when you had previously suspended your FEHB coverage to participate in one of these programs; when your membership in the employee organization sponsoring the FEHB plan terminates; under a non-Federal health plan.
When one of these events occurs, you may enroll, increase your enrollment, switch your designated family member under a Self Plus One enrollment when the current designated family member experiences certain QLEs, change your enrollment to another FEHB plan or option, decrease your enrollment*, or cancel your enrollment*.
*A cancellation may be made only if the enrollee can show that as a result of the QLE, he or she and all eligible family members now have other health insurance coverage.