/skin/common_files/modules/Advanced_Customer_Reviews/customer_review.tpl
Replace...
Code:
{$review.author|default:$lng.lbl_acr_guest}
With...
Code:
{$review.author|default:$lng.lbl_acr_guest|regex_replace:'/[^a-zA-Z0-9].*/':''}
That'll truncate the review author name after the first space character.