Hi Daniel,
As the conditional dropdown is a setting on the field itself there is not a way to build a checkbox on the form that would disable a conditional dropdown natively. Once set up if it needed to be reverted an administrator would need to go in and turn it off until it wasn't needed anymore. It is possible someone here on the forum will have a suggestion for how they do this or mimic doing this with a different set up (for example setting up a second set of relationship fields and hiding them until a checkbox is checked, which isn't optimal but could work depending on your volume and use case)
------------------------------
Evan Martinez
Community Marketing Manager
Quick Base
------------------------------