Generally speaking there are two kinds of such separators:
Actual method separators -- they appear before method/function body. I do not see them on your screenshot so you must have removed them already.
If someone needs it's controlled by
Settings/Preferences | Editor | General | Appearance | Show method separators.
Other separators -- before class/namespace/etc. Those are visible on your screenshot.
Thing is -- you cannot "remove" them in the same way -- only mask (make invisible). For that:
Editor | Color Scheme | General
Code | Method separator color
Either remove any assigned colors (try this first) or assign the same color as background.