I will try and walk you through this so that you can repeat the process for similar situations.
First open your query in design view.
On the left side of the query you should see. Field, Table, Sort, Show, Criteria. This depends on the version you are using. I am using 2003.
Under the Field that you wish to filter and on the Criteria line enter <> for Not Equal to.
Then right click in that field and click on build. Navigate to the List box. To do this open Forms. If it is open then select Loaded Forms, if not All Forms. Click on the Form you are searching for. In the next Column you should be able to find the list Box Name. Double Click on that. It should now show at the top of the Expression Builder. (The box you have open) Click OK. The expression should show in the criteria of the query.
Have a go.
If you run into problems post back with a clear description of what you did and what you think went wrong.
Or post a cut down version of you Database. The FAQs has instructions on how to do that.
But PLEASE attach a 2003 Version. I cannot help with 2007 or 2010.
If you can't convert then post anyway. Some one else will take over.