Method 1:
Open the Motion Tab
Select the object.
Click on the left-right arrows to go to the key you want to delete
Click on Delete Key/Position
Method 2:
Go to frame 0 (position A)
Open TrackView, select the Position track of your object, select all
Keys and delete.
Method 3:
Download the MKey utility by Anibyte.
Go to the frame with pos B and press the Delete button in the utility
floater.
When deleting in TrackView while the object is somewhere between A and
B,
it will remain at that position.
When deleting in Motion Tab, it will remain at the last selected key's position.