1. Put on higher gravity. 2. Put a roof on your maps. 3. Put a little lip on top of the walls that you can grenade jump up to. 4. Get rid of the grenades. 5. Reduce shields or something so if they try and grenade jump they die.
I build high walls and use ceilings selectively. Occasionally if there's a particular problem spot and I've run out of the usual scenery objects (boxes and walls), I'll use fence panels to make a kind of shutter that blocks jumping access to that spot. It's ugly but effective. I had particular trouble with one thing on my most recent map. It's a pretty small map with two bases and not much other room, and it was complicated by the fact that most of the neutral area is very dangerous and not a good place to put spawn points. To help counteract this and prevent people from getting pinned into their base by spawnkillers, I added a single box-sized "room" at the top of one base, with two spawn points in it. It had two side walls, a back wall, and a ceiling, so I thought you'd be forced to just drop out of it into the base when you wanted to leave the spawn. But in testing I found that you could grenade jump two different directions and clear the back or side wall of the base to get out of the map. So I used window panels and some scenery re-arrangement to add both 1. a shutter across the front, tilted down at a severe angle (so anyone jumping out forwardly would hit their heads on it); and 2. an extended side wall (jutting out a few feet) to block them from doing an angled jump toward the side of the base. It's effective - you can't go anywhere but down to get out of that spawn zone now. Dunno if this really helps you, but I'm sure I will be using this again for various applications.
Even high walls can be conquered pretty easily using brute shots (shoot the wall as you go up for extra height) or fusion coils. I wish we could make death barriers in forge =(
They can be annoying. The suggestions so far pretty much cover it. I would also suggest embracing the grenade jump as a part of gameplay and integrating it into your map design when appropriate, or at least not discounting the possibility.
Maybe you could use grav lifts on top of weapon holders that push downwards on top of the walls so if they try and grenade jump they will be forced away from the wall. :squirrel_chatting: