If a construction is incomplete, any passerby can just deconstruct it in a moment and wipe out all of the progress made on that site. The person doing this gains nothing by doing so, so it is an invitation to grief others.
Instead:
1. Allow the site to remain open, allowing for someone to come by and steal materials if they wish (and can get away with it). This at least is preferable to deconstruction.
2. Only the character that started a construction project, can deconstruct it.
3. As always, a GM can be requested to deconstruct a site (complete or not).