ax 2012 - Prevent wrong enum value in a form

Hi, i have a combobox in a form with three values, say "one","two","three".

I do a check on inventtable to find if the value i'm setting is ok for that item.

How do i take back the combobox to the original value once the user has selected the wrong one?

I dont know where is the right point to put the code.

Thank you in advance

regards