you can change PhysicalDesktopMode 1
in the /usr/NX/etc/server.cfg file
extracted part from the server.cfg file with explanation what each mode is doing:
# Set the interaction level for the session connected to the physical
# desktop:
#
# 0: View-only. The session is connected to the desktop in
# view-only mode, i.e. the user can't interact with the
# physical desktop.
#
# 1: Restricted. User connected to the physical desktop can
# interact with the desktop except for resize operations.
#
# 2: Interactive. User connected to the physical desktop has
# full interaction with the desktop.
#
#PhysicalDesktopMode 2