There is nothing much that can be done if someone doesn't pay you a bitcoin loan unless you signed a legal agreement.
No one lends 50-100 BTC these days. The people who haven't repaid these big amount loans probably took them when the bitcoin price was low.
No i am talking about bitcoin lending sites...like btcpop , btcjam, there is see people run away with 30-40 btc ..they had submitted id proof address proof there but still for months they are in collection and nothing happening...i also invested in such loans but not got my amount back which went to collections..