Should have to do with
aterm -bw 0 (as a command line)
aterm*borderWidth: 0 (as an .Xresources line)
but neither seem to work in flux or ice for some reason.
For flux, near as I can figure in that case, it's not doable with 0.1x and with 0.9x it'd be
in the ~/.fluxbox/apps file. There's also a 'toggle decor' bit (define an action in the 'keys' file) and a 'remember' item on the right-click titlebar menu but they seem to be remarkably stupidly thought out - kinda hard to get to the menu to 'remember the decor' when you've just 'toggled off the titlebar decor' that gives you the 'remember menu'.
(I'm sure there's an easy way to do it but I don't use flux and didn't feel like figuring it any further than the lines above.) So, yeah, try the apps file. The man page has stuff about most of what flux does.