not possible, false security.
use user logins and reputation instead. there's no technical solution, so you'll have to depend upon social solutions instead.
we've had flamewars about this sort of thing in the past over on quakeworld.nu - it got reverse engineered about 4 times (note: this is just the times that I know of), and trivially bypassed a few times too, it contained GPLed code violating copyrights, etc. There were at least two proxies that use a 'secure' client to authenticate for a cheat client. Its really just a joke.
which is basically why I cba verifying that sv_pure actually works properly.

Incidentilly, wall-hacks on an outdoors/heightmap game? are there actually going to be enough walls for a wallhack to really be particuarly useful? or are you just trying to get rid of those pesky retexturing/remodeling projects that tend to spring up from time to time...
there's nothing you can really do about aimbots either.
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