Delete all one-word comments

DISCLAIMER : Not a WordPress Developer, Just a MySQL DBA

If you have privileges to login to MySQL and query data you could collect all the comment_ID values that have one word.

SELECT comment_ID FROM wp_comments WHERE
REPLACE(TRIM(comment_content),' ','')=TRIM(comment_content);

You can test this by also seeing the comment_content

SELECT comment_ID,comment_content FROM wp_comments WHERE
REPLACE(TRIM(comment_content),' ','')=TRIM(comment_content);

Using the WordPress API, retrieve those comment_ID values and delete them.

Give it a Try !!!

CAVEAT

Other posts have been given for bulk deleting of comments (none of these are my posts):