keyboards are 2 layers of plastic film with tiny electric contact, separated by a third layer of plastic film with holes on the spot of every single key.
when you press a key, the top and bottom plastic are pressed against one another inside the hole of the middle layer, and the electronic signal going from the top to the bottom layer tells your computer that the key is pressed.
problem is: separation is the same as an average good quality book paper sheet, or maybe the same as the aluminum sheet that makes a soda or beer can. also, the electricity that goes trough your keyboard is extremely tiny, and sensors are VERRY sensible, so your air battery can lasts longer.
So even if you have a single hair touching those circuits, your keyboard would "break" the same way it breaks when you spill coffee or whatever liquid; even after the liquid dries, the coffee and the sugar are probably still there "pressing" your alt key.
solution: (this will probably void any warranty) open your computer carefully, remove the keyboard, dismantle all keys one by one, separate the plastic layers with surgical precision so you don't cause any ruptures to the electric "wires", then wash everything with a lot of water and very little mild soap, take extra care to remove ALL the soap, leave everything to dry in the sun (you know electronic components don't like humidity, right?) assemble everything together with "just-washed" oil free hands (don't touch the inner part of the sensors now).
Doing all this, if you have any luck your keyboard might work 100% again... or maybe you might make it stop working entirely, who knows? =)
if you have some money, then backup you computer, go to the applestore and tell them you want to buy a new keyboard, because you broke the current one.
they shall give you a brand new (refurbished?) one, and charge you only the new keyboard. or they might give the same one back to you, who knows? haha.
You can't setup Bluetooth devices without logging in, USB keyboard is the only way to go. Bring you Macbook to school or work and use one of their keyboards for a minute to logon and pair the Bluetooth keyboard.
Best Answer
I would reset both the System Management Controller and PRAM/NVRAM.
Reset the SMC
Instructions for resetting your System Management Controller (SMC) will depend on whether your MacBook Pro runs from a built-in battery or a removable battery.
As your question doesn't provide the exact model, use the relevant instructions below:
SMC 1 - For MacBook Pros, MacBook Airs, and MacBooks where you can’t remove the battery on your own (i.e. it's a built-in battery), here’s what you do:
SMC 2 - For MacBook Pros, MacBooks etc that you can remove the battery from, here’s what you do:
Reset your PRAM/NVRAM
Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). It may be worth you also resetting this. Here’s how to:
Note: When you log back in you may need to readjust some of your system preferences (e.g. mouse speed, time and date/timezone, etc).
Give these a go and let us know how you go.