You could program a workflow that is executed every now and then. This workflow would then take care of emptying the trash. Unfortunately, there is no automatic/standard/default solution to this problem. I checked the configuration files and I could not find a max trash-count setting either. However; if this is a huge problem and you need a quick'n dirty solution then you could do the following:
Find the template that the admin interface uses when it displays the "Are you sure you want to remove object xxx and its yyy children?" - This template contains a checkbox, "Move to trash", which by default is checked. Set this to unchecked and you should be fine (content will then be directly deleted instead of being moved to the trash).
