HOW TO HIDE WI-FI PASSWORD CHARACTERS?

How can I get my WiFi connection protected and the characters not shown when SHOW CHARACTERS in the laptops are checked??
I have a WiFi connection here, in our house and I don't want any other person to know my password. I once entered the key to my sister's notebook and a day later I was surprised when a bunch of her friends stayed in our house enjoying the FREE WiFi connection. I checked on her notebook and found out that when you put check on SHOW CHARACTERS, it really shows the password for my wireless connection. I want to hide the characters.
Help anyone?
