Hiding the SSID is minimal, at best. Are you saying that someone has been into your router configuration telnet pages and reset your router?
If so, then you should change your password immediately. Change it to something tough to guess and use some symbols in there. Something like SsD$89VBCa would be a good password that the average software will not be able to decode very easily.
If not, then I must have misunderstood where the problem lies.
Is is not possible to connect at least one machine on your network to the router via Cat 5 cable? If you could do this, then you could set your router to only accept configuration changes from a hard-wired machine. Just a thought.