Sometimes you do not need to specific customer but you can not delete it because there are many transaction on this item there is another solution to make this customer not chosen from the list and you can not choose it again do the following steps  :

 

 1- go to Accounts receivable tap in AX 2009

2- go to common forms

3- select customer details 

4- filter the form by your customer account

5-select the general tap

6-go to administration section in the general tap

7- and in stopped label choose from drop down list   All

 

Note:

it's by default no and no means you can work with this customer normally  

but all means we stop working with this customer and we can not choose it again from the list