Treyarch has added the option to disable auto mantle in Call of Duty Black Ops 4. Prior to the most recent November 20, 2018 (v1.07) update players were accidentally mantling objects left and right. Given the negative player feedback Treyarch decided it would be a good idea to add the ability to disable the feature.
To disable auto mantle go to Settings > Controls > Auto Mantle (On or Off)
The feature is most annoying when you are in a close quarters engagement. Majority of the time players do not mean to mantle like in this clip below of Dr DisRespect auto mantling mid-fight.
As of November 21, 2018 players are only able to disable auto mantle on Playstation 4 and Xbox One. However, the PC version is soon to follow.
Update: You can now disable auto mantle on all platforms. Treyarch released an update on November 23 that added the feature to the PC version of Black Ops 4.