6.4 A player must not endeavour to affect the running of any bowl, nor follow it up in such a manner as to obstruct the view of the opponent. For a first offence, involving the player’s own bowl, that bowl shall be forfeited. In the case of a further offence, the game shall be awarded to the opponent(s). The offender(s) to receive no further score and the opponent(s) to receive the maximum score.
Source: British Crown Green Bowling Association (BCGBA)