I fixed it like this.
1. make css folder in wordpress>wp-content>theme>[mytheme]
2. cp wordpress>wp-content>plugins>bbpress>templates>default>css>bbpress.css wordpress>wp-content>theme>[mytheme]>css
3. add these code lines in the css file.
#bbpress-forums fieldset.bbp-form input[id=bbp_anonymous_email]{ display: none; }
4. cp wordpress>wp-content>plugins>bbpress>templates>default>bbpress>form-anonymous.php wordpress>wp-content>theme>[mytheme]>bbpress
5. fix any email address in email textfield. Edit the form-anonymous.php
<input type=”text” id=”bbp_anonymous_email” value=”guest@guest.com” ….
