How to filter list box so it eliminate items selected before

[Don Quixote]

In my form there is combo box and list box and OK button
First I select I select training topic. Then I select, from list box ,employees who received this training .When I click on the OK button - those employees stored with the same topic of training in table(tblTrainingEmployees) and the form closed .
Now when I open the form again to add employees to this topic I want to eliminate those employees who has selected before . How can I do that ?.
Sorry for my clumsy English
Thanks in advance.

[jimatqsi]

Don Quixote, make a query that joins the Employee table to the TrainingTopics (all employees and all training topics. Then make a query that joins that query to the tblTrainingEmployees with a LEFT JOIN on employee and training topic. Include a WHERE clause that tests for a Null Employee number in the tblTrainingEmployees table. The result will be only Employees and training that are not yet matched in the tblTrainingEmployees table.

That query should be the rowsource for your listbox. After the training is selected, filter the listbox rowsource by the selected training topic.

Jim

[Don Quixote]

Thank you jim for your time.
I must confess that I am new to SQL .
Can You please show me the right syntax of those statements
thanks
Yoel