You can select this platform just with clicking onto it + press delete. Otherwise you should be able to select it in the scene outliner -> right upper corner of the viewport (brush is the name of it)
The lighting rebuild is from the landscape and object -> just click onto the big build button and then it should go away (the light gets build) or set your lights to moveable (dynamic)
Light builds are just here to build the lights and shadows -> they dont delete stuff ^^ You cant delete stuff in play mode, just in the normal viewport mode -> so in your case it is pretty strange that it just appears when you play the game. Check if you probably spawn it with the leve blueprint.
Otherwise upload the map somewhere and I will take a look at it