The Undo command (in the Edit menu) will reverse the pasting of the clipboard text.
When using this menu item in the wordprocessor, the Japanese text format will be automatically detected and the text converted during the pasting.
Note: If the text you are pasting in doesn't look right, it's possible that the format of the text is not being detected properly by JWM (this happens when text in one Japanese text format also converts correctly in another Japanese text format). To fix this, simply select the text format you think the text is in as the clipboard format (use the Set clipboard format popup menu at the top of the Text dialog). This makes JWM try that text format first.