You can try editing
dosbox.conf manually. The option you're looking for is in the
[joystick] section. Here's the example with commented options:
[joystick]
# joysticktype: Type of joystick to emulate: auto (default), none,
# 2axis (supports two joysticks),
# 4axis (supports one joystick, first joystick used),
# 4axis_2 (supports one joystick, second joystick used),
# fcs (Thrustmaster), ch (CH Flightstick).
# none disables joystick emulation.
# auto chooses emulation depending on real joystick(s).
# (Remember to reset dosbox's mapperfile if you saved it earlier)
# Possible values: auto, 2axis, 4axis, 4axis_2, fcs, ch, none.
# timed: enable timed intervals for axis. Experiment with this option, if your joystick drifts (away).
# autofire: continuously fires as long as you keep the button pressed.
# swap34: swap the 3rd and the 4th axis. can be useful for certain joysticks.
# buttonwrap: enable button wrapping at the number of emulated buttons.
joysticktype=auto
timed=true
autofire=false
swap34=false
buttonwrap=false
Just change
timed=true to
timed=false.