how do i filter dependents already selected in drop down list

[fras_too]

I've got a three tier dependent list set up for:
Division / Group / Line. (Thanks Debra Dalgliesh)

If a user selects a division, group and line there are no issues. However,
after selecting all three fields, I've had some users go back and change the
contents of the Division. When they do this - it makes the current content of
Group and Line invalid. Is there a way to 'blank out' the group and line
lists if the division list is entered/changed?

thanks in advance