Making objects non-mantleable
There is a spawnarg controlling whether objects can be mantled. It's called is_mantleable and it's 1 by default on most objects (except for AIs and some other objects that it wouldn't make sense to mantle), meaning that those objects can be mantled in-game. Setting it to 0 on an object will cause any mantle attempts on that object to fail.
Note that if an object is not mantleable, then objects attached to that object will not be mantleable either. For example, suppose you have an AI and a crate. The AI is not mantleable, but the crate is. However, if the AI were to pick up the crate then you wouldn't be able to mantle the crate either until the AI dropped it.
(For those not familiar with the term, "mantle" is basically a fancy word for clambering onto things.)