How to find out which plugin is blocking user?

Figured it out. It was the Stop Spammer Registration plugin. The way I figured it out is I did a text search in the wordpress directory. This found one file containing some of that error’s wording. I looked up the plugin by the folder name the file was in.

sudo grep -rnw './' -e 'You have been blocked from entering information on this site. In order to prevent this from'