So everything is deleted except for "BstkDrv_nxt.sys", "BstkDrv_arabica.sys" and "BstkDrv_bgp.sys". I can´t delete those because apparently they are running, nothing to be found in my task manager. Anyone got an idea? Can´t even install bluestacks again because "it´s already installed"

SOLVED:

1. Press Windows Key + R and type "regedit", click allow

2. Go to "HKEY_LOCAL_MACHINE"
   then SOFTWARE

3. Rightclick on everything Bluestacks related under "SOFTWARE" and delete

Now you should have Bluestacks deinstalled completely (and be able to redownload if you want.)