Danh,
I usually insert the scissor symbol at the beginning of the line (Insert -
symbol - choose wingdings - select scissors) then type my dotted line.
If you need a full line you can select the scissor symbol and lower it's
position using Format - Font - second tab - choose a lower position and hit
OK. It will look asif the scissor is cutting right on the line.
There is also a clipart available with a scissor cutting on a line, in the
clipart task pane, next to search for type scissors. You will find it there,
but be careful you can easily miss it as the thumbnail only seems to display
a line.
Luc