Nicola Puddicombe was sentenced to life imprisonment in 2009 after she was convicted of first-degree murder in the killing of Dennis Hoy.
The prosecution argued that Puddicombe conspired with lesbian lover Ashleigh Pechaluk to kill Hoy in 2006, about a year after the two women become romantically involved.
Hoy was bludgeoned with the blunt end of an axe as he slept in Puddicombe's basement.
Pechaluk was acquitted of murder in a separate trial.
As usual, the justices gave no reasons for refusing to hear the appeal.