If walls are under level 8 then they can't be upgraded with elixir. I have the solution for under level 8 walls ONLY. And here it is:
If walls are under level 8, then allow them to be upgraded with elixir only if the storages are at least half full. This will mean elixir can't be dumped into walls, only if it is overflowing and almost or past the loot steal cap. People can't upgrade walls randomly, but if they are maxed, then they can. Level 8 walls and up will remain the same and be able to be upgraded by elixir either way.

