Agency Threatening to put us on hold

My wife and I are wanting to put in a 30day notice for two boys we've been taking care for almost a year. Their behaviors are starting to get real bad and starting to affect our AD, they are also likely to go back to their bio dad soon and we would like to start moving toward adoptive placements only. We told our agency and they are threatening to put us on hold if we do a 30 day notice and I was curious if this was normal or if they are just trying to bully us.