Not having certain field values be entered in a DB save depending on the selection of other fields

I have a dropdown that if you don't meet the criteria of that dropdown it will show another textbox where you can fill out a custom value.

I want it so that if you select the value that shows the hidden custom field, that the dropdown will not save its value in the database.

What's the best way of doing that in Chronofroms V6?

In the event that hides the field, also disable it.