Is it possible to delete only the lateral edges in a excel matrix?
In the attacched image you can see tha matrix and you can notice that is almost impossible to delete left/right formatted edges without deleting up/down ones.
Select the cells that contain that edge, then go to Cell Formatting, then go to Limits.
There, you will be presented a rough draft on how the cells' borders are done. In your case, just click once on the middle border to disable it.