Bypass transparent proxy

[sephiroth]

hi,

i have question about transparent proxy. I read the manual in 
http://benzedrin.cx about transparent proxy with squid. I have network 
with 20 client connect to internet and i have implement that manual in 
my server. I want my client only use proxy in my server. My question is 
why the client still can bypass my proxy using another anonymous proxy.

Thanks and sorry for my english :D