SAM110041 | Repayments: issue repayment: cancellation of a repayment (Action Guide)
From HM Revenue & Customs · Self Assessment Manual
To cancel a repayment follow steps 1 - 5 below.
Also see section 'Recover Over-Repayment' where you need to recover an over-repayment from the taxpayer or a person who has received the repayment in error.