Disabling just a single key on Windows is possible, but it is highly unlikely that a computer technician could have gone to Control Panel to remap the keyboard.
More likely, one of the keyboard switches is broken, and the A letter is not working anymore. This happens quite commonly, and when the problem occurrs, the keyboard will be replaced.
You can find out if problem is hardware or software going to CMOS settings, and chosing to set a password. Then setting BIOS password AAA. If the A letter works in there, then the problem is software, and it can be fixed reinstalling Windows, if A key does not work in BIOS settings, then the keyboard must be replaced.
The keyboard for an Acer 5315 is not an expensive part, and can be purchased for less than $20.
Acer Aspire 5315 Keyboard Laptop Notebook replacement $15
Acer Aspire 5315 Keyboard Replacement
If a repair job had recently been done, and the laptop came back withthe keyboard problem, try calling back the repairman, and insisting for having the repair done at a reduced fee.