After pressing dismantle there should be something that pops up to ask if you really want to dismantle the structure.
I have not dismantled anything by mistake yet, but i did destroy a couple things by mistake before dismantling was added.
Agreed! I did put in a bug report for a confirmation "Destroy" box a while ago, and they said they were looking into it. I believe the only person who can destroy items is the person who created the tribe. My partner has all the same option in the tribe options yet she doesn't get the destroy option when clicking on buildings. Only I do. And of course I destroy a few before I got very very careful around buildings LOL.