You can't simply dispose of the belongings but have to give them a chance to claim them. Send a written notice, some way you can prove delivery, stating that if  they do not reclaim their belongings in 30 days, they will be considered abandoned and disposed of. Send a follow up after 15 or so days, also some way you can prove delivery. If after, say, 35 - 40 days (give them a few extra), they've not come for their belongings, you should be able to dispose of them.