Well, remember that spammers are not identified by their ISP (or country). Even worse, if they're just bots they'll appear as totally normal users with standard ISPs.
It is considered as an error to handle spammers by IP or ISP. You should use more powerful tools like login limitations or traps on your registration pages.
One simple trick is to use this html in registration page:
Quote:
<div id="countermeasure"><label for="email_"></label><input type="text" name="email_" /></div>
<script type="text/javascript">document.getElementById('countermeasur e').style.display = 'none';</script>
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After that simply check if your user is a bot:
Quote:
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<?php $is_a_bot = !empty($_POST['email_']); ?>
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The trick behind this is simple: for a normal user the field will be hidden and therefore they won't fill anything into it, a bot will fill in the field and you can detect it by the caused error and kick it or send it to fake pages.
NOTE: This sometimes causes small errors with the autocomplete feature of browsers, you should handle it by adding autocomplete="off" to your form.
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If the spammers are human idiots who just want to bother you you can IP ban them and send them cookies that will detect if they changed IPs. But ISP blocking them would be insane, blocking millions of ISP clients because of one idiot would be a extreme measure.
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Adding a country-wide block is just as bad as an ISP block. It would just help if you had legal restrictions in that country and would need to avoid people visiting the site.