You can access this tool from the following:
The Remove Boundary tool removes a boundary element from an unruled terrain model. This is useful when the boundary is causing issues when merging or manipulating the terrain model, or when creating a merged, as-built terrain model.
There is only one option when using the tool, creating a break line. If the original boundary element has valid elevations and the user wants to keep that data to maintain the integrity of the original data (i.e., survey data) then use the break line option. If there were no elevations or they are not relevant to the terrain model, do not include as a break line.
The tool can only be used in the active file containing the terrain model, as write access is needed to modify the terrain model. It will not work if you are in a file where the terrain model is just referenced.