Someone has been using my Wireless Broadband

One of my community councillors thinks I'm quite good with computers. I can however be a little thick. We have had this occasional problem accessing the internet. Its very frustrating because it goes then comes back. Anyway, I have a wireless Belkin router.
We do not use the wireless bit at present, we are all hard wired.
For months I have been wondering if the Police were downloading my hard drive or we were being used in some DNOS denial of service attack as a "BOT"

I wouldn't mind but someone has been hogging all the bandwidth.
We have nearly lost money on Ebay auctions.

It would appear my wireless side of the router is open. This is done as a default. I think this is wrong. I ran an IP address that was in the firewall log.
It's in China ..............................
I live next door to a Chinese Take Away.

I have looked at the wireless security before.
It looks complex.
There are 4 versions
The fear is being locked out by the firewall without the encryption key.
Anyway my friends next door appear to be sorted.
I've set the encryption to mild.