Hi,

You are trying to use wrong tool for dimentioning. You are using Parametric dimensions, but you need DIM command:
DIM (Command)
http://help.autodesk.com/cloudhelp/2016/ENU/AutoCAD-MAC-Core/files/GUID-45C1A271-9650-4927-858F-B3BD...

In order to delete dimension constraints you already created use menu Parametric -> Delete Constraints, when you will be prompted to select objects, type ALL at the command prompt and hit Return.

Maxim