After 2.5.3 theme update you can simply delete the SUBJECT line from Theme Options -> Email Management and the email is not sent.
Starting 1.21.1 the email management supports simple HTML code.
You can find the Email Management section in WpRentals Options-Advanced- Email Management
Use text mode and <br> tag for new line
IMPORTANT: <br> can be added only in Text mode. If switch to the Visual mode from text editor it is removed.