Session table exploding due spam sessions

Hi,

I've got a comment spam problem. Mollow does a great job in keeping out the spam but the problem is the session created by the spammers. My session table is exploding (currently 600 MB). It's filling with sessions like below. Can anyone tell me a good way to handle this problem? I've allready cleaned out the session table and deleted everything that is older dan 2 weeks but in just 3 days, the table has grown by 300 MB. Damn spammers...

Thanks for any help,

Ward

Session example:

messages|a:1:{s:5:"error";a:1:{i:0;s:71:"Your submission has triggered the spam filter and will not be accepted.";}}spam_form|a:30:{s:4:"name";a:24:{s:5:"#type";s:9:"textfield";s:6:"#title";s:7:"Uw naam";s:10:"#maxlength";i:60;s:5:"#size";i:30;s:14:"#default_value";s:7:"Anoniem";s:9:"#required";b:1;s:5:"#post";a:7:{s:4:"name";s:10:"Ezzxhyliqp";s:4:"mail";s:20:"libbvffzlx@gmail.com";s:8:"homepage";s:62:"http://www.lowriderhaven.com/forums/m_3726/mpage_1/key_/tm.htm";s:7:"comment";s:3538:"<a href="http://irs.aau.org/?q=node/7479">car cheapest insurance quote</a>
<a href="http://bkfuller.com/members/Download-The-Hottest-Ringtones.aspx">country music ringtones</a>
<a href="http://ishug.is/members/Get-The-Hottest-Ringtones-For-Your-Mobile-Phone.aspx">mobile phone ringtone</a>

SNAP, this is followed by a huge list of URL's

";s:13:"form_build_id";s:37:"form-b2a8344f084c69f7908a5b8ac1373a4d";s:7:"form_id";s:12:"comment_form";s:2:"op";s:4:"Save";}";type|s:7:"comment";