There are several tutorials and forum posts regarding this issue, we will try to keep it simple and short, less reading same results.
The wordpress native comments system for your posts and pages contains a field called website. Users are supposed to fill out this field with their website. Although admirable, unfortunately 99% of cases this field is being used by spam users and spam bots. Quick easy fix is to remove this field from the wordpress comment system.
How-to remove the website link field from wordpress native comments
Just navigate to your WordPress Administrator, locate and edit your template functions.php file. Appearance -> Editor
Add these lines at the bottom of the file: