In some cases, having asking people for their website is very useful not only as a way to create backlinks between two sites, but to also make your website obtain more traffic. However, this can also be a target for spam websites that may hurt your website ranking. You cannot ask people to not include their website in making comments, so remove the url field instead.
The disadvantage of removing your url field is that you will get fewer people to comment in your blog as they need this field to build backlinks. However, if you need to filter authentic readers of your site, this is a great way to go.
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