As far as I know there, is no specific rule about that. You can guard as close as you want as long as you aren’t breaching any of the following rules:
12.6. A player in an established position is entitled to remain in that position and should not be contacted by an opposing player.
12.8. Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position.
12.9. All players must attempt to avoid contact with other players, and there is no situation whereaplayermayjustifyinitiatingcontact. “Making a play for the disc” is not a valid excuse for initiating contact with other players.
12.10.Some incidental contact, not affecting the outcome of the play or safety of players, may occur as two or more players move towards a single point simultaneously. Incidental contact should be minimized but is not considered a foul.
12.11.Players may not use their arms or legs to obstruct the movement of opposing players.