1) Yes, a softbox or whatever in front of it will somewhat decrease the output by absorbing a little light, scattering some away from your camera, and reflecting some back at the flash. Probably at least 1 stop for a large white softbox.
2) You could bounce it off the wall or ceiling. This often improves the look of flash anyway on the cheap, as well as decreasing its strength due to essentially doubling the range in most cases (+ scattering + absorption). It will as a rule of thumb decrease power by about two stops with a white ceiling or wall nearby. If you bounce off of a darker colored wall, then the drop in power could be very significant (several stops potentially), however if you bounce off of a non-white wall, your flash will pick up a color cast the same color as the wall. Gray and black walls are relatively rare.
3) You could trigger it remotely and then move it behind you so it's further from the subject. 40% further away = 1 stop weaker flash.
4) You could decrease your aperture. One stop smaller aperture = one stop effectively weaker flash.