Competitor Buying Products Off Me then Returning them

Hi

so a competitor on one of our items we sell brought 5 packs of an item that cost us nearly £20 to ship then returned them a few days after they receive them. we noticed via them having the label on the box having there brand name on it. should i do anything about this or just leave it? just found it strange they ordered such a large amount… i understand they are allowed to purchase items on amazon the same as everyone but very strange & costing us money… any comments would be appreciated.