Validation on account id of opportunity - Java script

Hi,

I am doing some validation on account of opportunity.  Is is validating properly when change account from lookup, but it will give warning and accept the changed account. System allow to move to next field.

I want to bring the previous account back to the account field if it is not a valid account until user input valid account.

Please find the java script below and help me to solve this issue.

 if (( found1 == 'n') || ( found2 == 'n') || ( found3 == 'n'))

          {
            alert("Please enter contact of IT, Purchase,Finance/Management on this account before entering opportunity");
            crmForm.all.customerid.SetFocus();
            event.returnValue=false;
         }

pnoushu@yahoo.com