If you are really determined to make the 8va line disappear, set the Minimum length for octave line (in Engraving Options > Octave Lines) to -11/128 spaces.
The following picture is a screenshot taken at maximum zoom (1600%). The artefact is discernible, but very small.
At normal size you would possibly not know it is there.
With some experimentation, you might be able to reduce it even further.