Remove payments from orderform

How do you remove payments from an orderform?

I have tried to use .RemoveAt() and .Clear() but the payments only seem to dissapear for a while and then shows up again.

I have also tried the following which just gives me a SqlException

foreach(Payment payment in orderForm)
{
payment.Delete();
}
orderForm.AcceptChanges();

Do anyone know or have any sugestions?