If you are using a router, you might be able to restrict access by using a MAC lock (nothing to do with Apple). You will need to find out the MAC address of your devices and input it into your router. I can see the MAC address of the devices connected to my Netgear router, so that's easy.
It's not foolproof though. MAC address can be faked. I have done it before.
All wireless routers I have used (been using them since 2001) allow for mac address filtering. Even the old ones would let you choose which currently connected mac addresses are allowed when you turn on filtering. So you could make sure everything is connected and enable filtering, select the allowed addresses and be good to go. I do this in my house and it only takes a few minutes once you get used to it (and I have blue ray player, 6 laptops, 2 Wii's, XBox 360, and 6 android devices)

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This combined with lunatic59's suggestion about encryption, secure passwords, occasional changes, etc. will make your network pretty secure.
As a side note to secure passwords, you can create secure passwords out of things that are easily remembered by you. Such as taking a favorite cars name and typing it in with your fingers shifted one or two keys to the left or right (e.g. GTO shifted two letters to the left --> DEU). Obviously, you would chose something greater than 3 characters, generally 8 or more is best.
Another technique I use is symbols, which to some seem confusing to remember. Here is a simple way to integrate symbols in a password. Let's say your favorite car is a 1969 Dodge Charger (sorry, I'm a Dukes of Hazzard fan

) you could simply hold the shift key when you type some or all of the numbers. Using all of the numbers would result in !(^( Dodge Charger. Which if you check in any password strength measurer (not a word, but you know what I mean

) it will rate as strong.
Again, these are ways to make extremely strong passwords out of simple to remember terms for you. Also remember if you combine two of these methods, shift keys and symbols, the password strength increase is exponential. In some cases you can keep your same password and change technique.
I know this is not a security blog, but I thought this might be useful information for some.