Can't remove lockring
If it screws on then it is a freewheel not a cassette.  They are not the same.  There is a link in my comment.  

Your spacer kit with one sprocket was probably for cassette / freehub.  It is not going to work.

I suspect you can find a freewheel single speed conversion.
You need a single speed freewheel - not a sprocket and a lockring.