my php guestbook gets spammed

for some reason a guestbook i coded in php gets spammed since about 1 week - app. 10 entries per day with content like this

cBlE3U
[URL=http://xxx-link]girls kissing girls[/URL]

[URL=URL=http://xxx-link]xxx words[/URL]

[URL=URL=http://xxx-link]xxx words[/URL]

[URL=URL=http://xxx-link]xxx words[/URL]

[URL=URL=http://xxx-link]xxx words[/URL]

i don't have the time to implement capatcha right now so here is a short work-around how i fixed it temporarily

original code

$id = time();

$name = addslashes($name);
$email = addslashes($email);
$homepage = addslashes($homepage);
$message = addslashes($message);
...

$pos = strpos($message,"[url");

$sqlcommand = 
  "INSERT INTO guestb (id, ...) VALUES ('$id', ...)";

mysql_query($sqlcommand) 
  or die("Error: " . mysql_error());

echo "thx for your entry";

modified code

$id = time();

$name = addslashes($name);
$email = addslashes($email);
$homepage = addslashes($homepage);
$message = addslashes($message);
...

$pos = strpos($message,"[url");

if ($pos === false) {
  $sqlcommand = 
       "INSERT INTO guestb (id, ...) VALUES ('$id', ...)";
    
  mysql_query($sqlcommand) 
      or die("Error: " . mysql_error());
}

echo "thx for your entry";