Mist is actually not directly connected to the blockchain or your account. It's just a UI over top of the actual Ethereum client.
If you look on your mac you should see the folder
~/Library/Ethereum. It actually contains the blockchain and your account keys in the
keystore sub-directory. DO NOT DELETE THE FILES IN THE KEYSTORE DIRECTORY they are the keys to your account. If fact you should back them up to a secure place.
You can delete Mist simply by deleting the Mist folder. But I wouldn't delete the
~/Library/Ethereum folder unless you want to download the entire blockchain again.
As for your original password. If you lost that, you're pretty much out of luck.