here is what I suggested this morning on french XOOPS forum.
for what i know, this spammer always uses a particular url in his profile.
i modified my register.php (xoops 2.0.16) to add a test on this url.
backup your register.php before any change.
line 41, replace :
 function userCheck($uname, $email, $pass, $vpass)  
with
 function userCheck($uname, $email, $url, $pass, $vpass)  
line 141 :
 $stop .= userCheck($uname, $email, $pass, $vpass);  
with
 $stop .= userCheck($uname, $email, $url, $pass, $vpass);  
line 174 :
 $stop = userCheck($uname, $email, $pass, $vpass);  
with
 $stop = userCheck($uname, $email, $url, $pass, $vpass);  
to finish, line 92, after :
 if (strrpos($uname, ' ') > 0) { 
    $stop .= _US_NICKNAMENOSPACES."
"; 
}  
add :
 //spam gwb 
if (preg_match("/george-walker-bush/i", $url)) { 
        $stop .= _US_INVALIDMAIL."
"; 
    } 
//spam gwb  
thus, if someone try to register on your website, with an url containing "george-walker-bush", a message will tell to him that his email is not valid.
this is surely a temporary solution, but this prevents your website to be spammed easily !