Hi i have a question about a LookUp in a Table/Form.

I have a lookUp reverence to an other table when these records are connected i want the lookup to stop giving this(the value if already used) value to connect to an other record.

In other terms...
Form1 - Value is entered.
Form2 - Value must be connected before it can be completed.

Bud when i go to the Form2 again i can connect the data if already connected to a record in Form1 to an other record in form 1 again...

I want this disabled...

Hope some1 can tell me how...
Thanks