Hover over menu item and press desired keyboard combination. To remove one, hover and press Backspace. If it doesn't work, see the FAQ for pointers how to enable GTK+ shortcuts changing for you may not have it enabled. Attached screenshot from the FAQ as well.
From http://docs.xfce.org/faq, quoting here for your convenience:
A number of Xfce applications (Xfce Terminal and Thunar for example)
support the standard GTK+ way of changing shortcuts: simply hover over
the menu option with the mouse pointer and press the keyboard shortcut
you want to rebind it to.
To delete a keyboard assignment, press the Backspace key while you are
on the menu entry.
Just had same problem and found this out, I did that by accident with s and could not type commands with s cause it would ask me for a new terminal title. :D
EDIT after comment:
if the shortcut doesn't change, then you need to enable the feature in
GTK+. This can be achieved in 3 ways:
If you are running the Xfce desktop environment, enable Editable menu accelerators in the User Interface Preferences dialog.
If you are running GNOME then you can enable Editable menu accelerators in the Menu and Toolbars control center dialog.
Otherwise put the following in your ~/.gtkrc-2.0 file (create the file if it doesn't exist):