How to remove custom ships and parts from the game?

Hi,

I tried to clean up my game and delete some shipdesigns and parts I´m not satisfied with anymore. No matter what I do this ships and parts come back again as soon as I close my game. I already reinstalled the game, enabled and disabled steam overlay, deleted them before starting game, while game was running and after I closed game. Result is always the same. Renaming them doesn´t help either, it just makes it worse because I have now several duplicates.

So it would be great if someone could help with this problem. Designer is getting messy and realy need a cleanup...