If you have not modified the block template then you should be able to make this change by updating line 50 to be like this:
<th style="font-family: 'Times New Roman', Times, serif; font-size: 10px;width: 134px; text-align: right; padding: 3px 10px 3px 3px; color: #666; font-weight: bold;"><?php echo(str_replace("Registration group","",str_replace("_"," ",$pkey))); ?>:</th>
The only modification is to add in another str_replace around the echoed value. I added a str_replace to find the "Registration group" part and replace it with nothing. This should remove it from your email message.
You should be able to find this file in webassist > email > templates.