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You can delete the applied validation rule using the IValidation.Delete of the IValidation .
Refer to the following example code to know how you can delete validation rule applied to a cell or a range of cells in a worksheet.
//Add validation
worksheet.Range["A1:A3"].Validation.Add(ValidationType.Whole, ValidationAlertStyle.Stop, ValidationOperator.Between, 1, 8);
worksheet.Range["B1:B3"].Validation.Add(ValidationType.Whole, ValidationAlertStyle.Stop, ValidationOperator.Between, 11, 18);
//Delete validation.
worksheet.Range["A1:A2"].Validation.Delete();