If someone has made the password to your WLAN public knowledge then it is a problem for more than a couple of reasons. The first being that they have affectively free reign over your internet connection, which you are paying for and they are not!! It doesn’t sound like you have any control over what they can do either so in the unlikely event that they do start illegal activity using your connection then you are liable for it as its your connection. Also performance, you might not be able to stream that all important last in the season program because they are using your connection for heavy downloads.
My advice would be to reset the password so that once again you are in control of the connection and who uses it.