The first thing to do is to define the rule. For example, if you consider that you send three messages exactly, can be very easy to bypass the system.
One suggestion I had used for something like this - is this function http://php.net/similar_text
$var_1 = 'PHP IS GREAT';
$var_2 = 'WITH MYSQL';
similar_text($var_1, $var_2, $percent);
echo $percent;
// 27.272727272727
similar_text($var_2, $var_1, $percent);
echo $percent;
// 18.181818181818
Then check the user history, if the last 3 messages had 80% or more similar you block the user.