TCP is a 2 way street. You need to allow the telnet server to send replies back to your hosts. That said, you could apply an inbound acl to the inside int of your pix to limit what goes back:
Assume the remote network is 192.168.0.0/24
Assume the remote telnet server is 192.168.0.5
access-list 105 deny ip any 192.168.0.0 255.255.255.0
access-list 105 permit tcp any host 192.168.0.5 eq 23
access-list 105 permit ip any any
this should do what you seek:
line 1 blocks all traffic to their network.
line 2 only allows tcp traffic going to port 23 (telnet) of their telnet server
line 3 allows all traffic that does not match 1 or 2 to pass - this allows all of your other traffic to pass, just like it would before you started restricting outbound data flow (pix default is to allow all out, and block all in)