Deleting imported curves

i have a 3d scanned model im trying to delete a load of imported curves. I want to just be able to select box and delete the areas I dont want as its such a huge file the computer is struggling!

Once ive done this I will make a surface model of it, so shall I convert the curves into a sketch then try to split line to break it up?

computers running SO slow its unreal

thanks,

Andy

SolidworksSketching