[Plugin: Contact Form 7] Delete a required field

My form starts with a selectbox. If a certain choice is made, i want to remove a required field.
I do this by emptying a container.

When i remove my element and submit my form, the field is still being validated. Is there a way around this?

Thanks in advance,

Rick

http://ww.wp.xz.cn/extend/plugins/contact-form-7/