When you get to the main server on a LAN, the sysadmin will start tracing you as soon as you connect. The trace tracker starts going off and the LAN map starts showing the sysadmin's progress in tracing you. However, the two are not the same. On missions where you have to destroy the targets system's files, go into the console and type:

cd usr [press enter]
delete [press enter]

Then while it is deleting files type:

disconnect [press enter]

You can now switch to the LAN view and watch the admin, because the machine will not run the "disconnect" command until after it is done deleting. When the files finish deleting, you will be automatically disconnected, and since your watching the sysadmin you can hit the "Disconnect" button when he gets too close or the trace tracker runs low on time.