Disable Workstation in Intranet

I run a XP Pro and I have a small workgroup at my home with 4 computers on
it for my family. Whenever my sons use it too much, it became very slow for
my own and I used to disable their connections till my work is finished. I
do this by cliking disable on network connections screen.When I finish my
work I resume their connections.
Latelly, when I tried to follow the same standard procedure, a note showed
in the monitor saying :
"It is not possible to disable the connection at this time. This connection
may be using one or more protocols that do not support Plug-and -Ply or it
may have been initiated by another user or the system account. "
I do not ustand a word about this note and I have not done any different
procedure.
Anyone can explain it to me and guide me how to resume to the previous
situation when I disable the connections at my option ??
Thanks in advance