For those who don't want to compile anything and only use stock programs :

xprop -name 'My window name' -format _MOTIF_WM_HINTS 32i -set _MOTIF_WM_HINTS 2

Replace My window name with the name of your window, or you can omit the -name option, and the program will ask you to click on the window where you want the decorations removed.

In fact I didn't know how to do that, until I had a look at Muktupavels' C program ;)

For those who don't want to compile anything and only use stock programs :

xprop -name 'My window name' -format _MOTIF_WM_HINTS 32c -set _MOTIF_WM_HINTS 2

Replace My window name with the name of your window, or you can omit the -name option, and the program will ask you to click on the window where you want the decorations removed.

In fact I didn't know how to do that, until I had a look at Muktupavels' C program ;)

EDIT : This isn’t guaranteed to work. It will work with all window managers accepting Motif-style hints, such as GNOME 3 (and probably 2 though I didn’t check), and mwm of course (anyone still using that ?), but for example, as a comment below pointed out, it doesn’t work with XFCE. Unfortunately, I don’t think there’s a method that works with all window managers.

I changed the property type in the command line from 32i to 32c (32-bit cardinal, i.e. positive integer), which is the correct type for that property.