Australia Rugby's excellent Refereeing in Practice guide has a section on this:
Teams deciding not to engage the maul at lineout
# If the defenders in the line out choose to not engage the line out drive by
leaving the line out as a group, then PK to attacking team.
# If the defenders in the line out choose to not engage the line out drive by
simply opening up a gap and creating space, and not leaving the line out,
the following process should be followed:
# The attacking team would need to keep the ball with the front
player if they were to drive down-field (therefore play on, general
play – defenders could either engage to form a maul, or tackle the ball
carrier.)I
# If the attacking team immediately passes the ball back to a player
behind the front player or at the rear of the group, the referee would
tell them to “Use it” which they must do immediately.
# If the team drives forward with the ball at the back (and ignores
the referee’s call to “Use it”), the referee should award a scrum
to the defending team for “accidental offside” (rather than PK for
obstruction).
Here's the link [PDF]