By default ClusterManager uses NonHierarchicalDistanceBasedAlgorithm, that doesn't implement removing elements.
Try to use GridBasedAlgorithm instead (it supports elements remove):
clusterMng.setAlgorithm(new GridBasedAlgorithm<MyClusterItem>());
Or, for better performance, wrap it with PreCachingAlgorithmDecorator, as ClusterManager does by default:
clusterMng.setAlgorithm(new PreCachingAlgorithmDecorator<MyClusterItem>(new GridBasedAlgorithm<MyClusterItem>()));
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