There isn't an option to change the way Radiant modifies brushes based on your action as far as I know. Wish there was
The solution I'm afraid is to get used to it. But by that I don't mean 'suck it up'. No, what I mean is learn to anticipate what Radiant will or won't try to do to your brushwork, then work out the sequence of actions you need to perform to end up with the shapes you want without going down a series of actions that leads to hideous brush mutilation*
For the brush shapes in your screenshot, the sloping underside should have been the last feature you added to them. It would have been best for the top and bottom surfaces to remain horizontal up until then. Always try to create surfaces that slope across more than one axis AFTER any other brush manipulation.
Your best ally for such surfaces is the three-point clipper. It allows you to create many of the sort of shapes in your screenshot without any vert moving at all. Can take a bit of practice to get your head around cutting in three dimensions, but it does work rather well.
There's also Edge Manipulation, which is simpler but much more reliable. Endeavour to make each action you perform on a brush as simple and axially restricted as possible, so move edges instead of verts when you can.
To sum up:
think through how best to get from initial brush -> brush you want
use edge manipulation first where possible
use the clipper second where possible
use vertex manipulation last, and try to move as few verts as possible and in only one axis
Lastly: your taskbar looks eerily similar to mine. Please stop it.
HTH
