How about disabling your serial port?
Download and extract http://www.yeahware.com/download/enable.zip (http://www.yeahware.com/download/enable.zip). Place the Enable.exe in the same folder as the script below.
Just change the device= line in the script to match the start of the device to toggle, and schedule the script to run every night and morning.
Skrommel
;ToggleHardware.ahk
; Enables or disables a hardware device
; Download and extract Enable.exe from http://www.yeahware.com/download/enable.zip
;To run this script download AutoHotkey from http://www.autohotkey.com
;Skrommel @ 2008
#NoEnv
#SingleInstance,Force
SendMode,Input
;Change this line so it matches the start of the device to toggle
device=Comm
Run,Enable.exe
WinWait,Enable and Disable Devices
WinActivate,Enable and Disable Devices
WinWaitActive,Enable and Disable Devices
Send,%device%
Sleep,1000
Send,!ce
Sleep,1000
Send,!cd
WinWait,Change Device Status
WinActivate,Change Device Status
WinWaitActive,Change Device Status
Send,{Enter}
Sleep,1000
WinClose,Enable and Disable Devices