Try to set property "tabEnabled" to false for the combobox: combobox.tabEnabled = false;
If it is placed in a seprate movie also try to set "movieclip.tabChildren = false;" if above does not work.
That was my previous solution. However I think I know what the issue was. I was assigning a value that was not in the list in the combobox. Only issue now is that when you tab to it, it doesn't seem to highlight.