How to avoid this?

[tonibarcelona]

How about what you had before, but

  if( ereg("\?alpha_filter_id=[0-9]+$", $_SERVER['REQUEST_URI']) ) {
    header('HTTP/1.1 301 Moved Permanently');
    header('Location:http://www.mysite.com/shop/');
  } 

That will catch the filter if it is the only variable. I think that is a brutal hack, but it's up to you.

Personally I think that you are worrying about nothing, in time google will get a decent index of your site and those things will filter away.

Many thanks for your proposal. Maybe it´s abrutal hack, but by now is the only i have... At what fileis better inserting this? Filter.php? index.php?, or maybe to make a new php file and hooking it at auto-load?

I worry about this because i have 23 "main pages" indexed at google with an alpha filte id!!! Very bad for the serps... That thisn´t happen to any other zen cart user?