True, but I've had my share of problems with ZoneAlarm as well. It has proven to be marginally compatible with several software packages (such as Cisco
's VPN client), plus it takes a while to configure properly and tends to pop up a lot of annoying messages.
Not only that, but it's likely that the kids have admin rights on the machine. ZoneAlarm will make it obvious where the block is. IPSec rules are quite a bit harder to trace down, if you don't already know all about them. :-)
Of course, they may very well just switch to a different IM client, then you block the port, back and forth. At some point, you just have to go and physically kick 'em out.