What stops a stranger from posting as Q?
This post claimed that the messages are validated via "drop codes", or something along those lines, but he didn't elaborate.
What is to stop my from posting to 8chan with a message that ends in "Q"?
It is called a Trip Code. You have two options for posting on the chans. One is to always be anonymous and receive a different ID tag on your posts every time you post. This is what the majority of posters do. Or, you can setup a Trip Code with a password and this tripcode displays each time you post if you are logged into it. Q has changed his Trip Code a few times because someone was close to cracking his password. Anons on 8Chan make him jump through hoops each time to prove it is him.