removing foreign characters from database

I’m getting collation errors on my site like this:
[07-Dec-2014 16:33:22 UTC] WordPress database error Illegal mix of collations (latin1_swedish_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation ‘=’ for query SELECT comment_ID FROM wp_comments WHERE comment_post_ID = 2084 AND comment_parent = ‘0’ AND comment_approved != ‘trash’ AND ( comment_author = ‘非常にアクセサリー ショルダーバッグボストン’ ) AND comment_content = ‘非常にアクセサリー ショルダーバッグボストン\n\nWhat Defines Israeli Parenting?’ LIMIT 1 made by require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), include(‘wp-trackback.php’), wp_new_comment, wp_allow_comment

I’d like to avoid having to switch all of the collations to utf8, as I don’t actually need any characters other than latin. These comments are all spam anyway. I’ve emptied my comment trash folder–could it be that these comments are still there somewhere? I’ve searched the database for the character string without success.
And another question–why is the site calling up these comments in the first place?