You can undo (cancel the recording or the last overdubbing) to erase the phrase you just overdubbed.
RC-5
Press the pedal for two or more seconds during playback/overdubbing to undo. Press the pedal again for two or more seconds to redo (cancel the undo).
RC-500
Press the [TRACK SELECT] switch to select the track, and long-press the [REC/PLAY] switch (for two or more seconds) during playback/overdubbing to undo. Long-press the switch again to redo (cancel the undo).