To break a hanged R process from open source R, you can hit Ctrl-C. However, when you hit Ctrl-C from the RRE console in Windows, you get this message:
"@Value cannot be null. Parameter name: text"
From the RRE console, you can hit the ESC key to break out of the hanged process or long running query.
In general there's no guaranteed way to break out of a long-running query or R command & usually the only way to interrupt it is to kill the RevoIDE process using the task manager in Windows.