Currently with geometry builder, you cannot fix or lock an element in position. So the builder is a list of connected elements and if you edit any of the elements higher in the list it changes the position of all below that element. If you give the ability to fix(lock) an element in its position, it means editing any other element won't adjust that elements position. For example, you have to maintain a bearing and offset from a structure ,this will help you achieve that, by locking a line elements position

Can you please add more information? What do you mean by "fix" and what do you mean by "certain elements?" When and why would this idea be helpful? What task are you trying to accomplish?