textFlow.addEventListener(FlowOperationEvent.FLOW_OPERATION_BEGIN, opBeginHandler);

public function opBeginHandler(evt:FlowOperationEvent):void { 
    var flowOp:FlowOperation = evt.operation; 
    if(flowOp is UndoOperation) {
        //user has the option to cancel undo operation
        if(!verifyUndo(flowOp.textFlow) && evt.cancellable) {
            evt.preventDefault();
        }
    }
}
private function verifyUndo(tf:TextFlow):boolean {
    //verify whether user wants to undo this operation
}