Procedures for ending Social Security Participation are described in 20CFR 416.1333, Subpart M.  The law is repeated here for your benefit:

SUPPLEMENTAL SECURITY INCOME FOR THE AGED, BLIND, AND DISABLED
20 C.F.R. Part 416 SUBPART M--SUSPENSIONS AND TERMINATIONS

Sec. 416.1333 Termination at the request of the recipient

Termination at the request of the recipient A recipient, his legal guardian, or his representative payee, may terminate his eligibility for benefits under this part by filing a written request for termination which shows an understanding that such termination may extend to other benefits resulting from eligibility under this part. In the case of a representative payee there must also be a showing which establishes that no hardship would result if an eligible recipient were not covered by the supplemental security income program. When such a request is filed, the recipient ceases to be an eligible individual, or eligible spouse, effective with the month following the month the request is filed with the Social Security Administration unless the recipient specifies some other month. However, the Social Security Administration will not effectuate the request for any month for which payment has been or will be made unless there is repayment, or assurance of repayment, of any amounts paid for those months (e.g., from special payments which would be payable for such months under Sec. 228 of the Act). When the Social Security Administration effectuates a termination of eligibility at the request of the recipient, his legal guardian, or his representative payee, notice of the determination will be sent in accordance with Sec. 416.1404, and eligibility, once terminated, can be reestablished, except as provided by Sec. 416.1408, only upon the filing of a new application. [42 FR 39100, Aug. 2, 1977]