rental agent - non performance

If an agent has done a horrible job managing a rental property. Then how can you get out of giving them the 90 Days notice that you have found a decent Agent?

Any ideas? My Parents are cracking it on their Gold Coast visit lol. The Agent told them he is not giving the key's back and to find a lawyer or pay them $340 for not Providing the notice.

Mums plan is change the locks and again tell them to stick it.