EDIT:
Please visit here:
Flex textArea remove/replace invalid little squares
Seems to be the issue & need to find a solution for this.
OR
I was able to handle it by using the textArea changingHandler to allow only certain characters:
if (event.operation is PasteOperation)
{
event.preventDefault();
var txt:String = Clipboard.generalClipboard.getData(ClipboardFormats.TEXT_FORMAT).
toString().replace(/[^A-Za-z0-9\s$%&*!@-_().]/ig, "");
......
}