How to Disable Left Command key
One thing is remapping a key.  It's another thing altogether to ignore a key that's a modifier key.  This is not the biggest issue, however.

Short answer:  You need to take it in for repairs and at the very minimum have it cleaned with an ultrasonic cleaner or some sort of solvent for electronics.  

The problem is not the alcohol in the beer, but the sugars and other ingredients that can short out components.  You are already seeing that because you have shorted out the electrical contacts for the Command key making it appear it's pressed.

This is not a "software fix" per se because the issue is a physical one - you have physically shorted the connections so no matter what happens, the system is going to see that key pressed because, it is.

You will most likely have to get the keyboard replaced.