RewriteCond %{HTTP_REFERER} !^http(s)?://(www.)?paypal.com [NC]
RewriteCond %{QUERY_STRING} ^action=register$
RewriteRule ^wp-login.php index.php [NC,L]

RewriteEngine On
RewriteCond %{HTTP_REFERER} !^http(s)?://(www.)?paypal.com [NC]
RewriteCond %{REQUEST_URI} ^/your-registration-url$ [NC]
RewriteRule ^your-registration-url$ https://website.com/404 [R=301,L]