This is covered in the annotation to rule 8.5:
"If other players are still moving towards the disc (a common situation after a turnover), the closest player can decide not to pick up the disc – this is not in itself a breach of the rules. If no player on offense has been walking towards the disc since the turnover occurred, this is a breach of the rules. It is recommended that the opposing team provide a warning to the opposition and make sure they are aware of the rule before enforcing any time limits."