Self Destruct code

Hi I have a funny request; I wrote a system for a client and am rather concerned that I am not going to receive payment for the work done. They want me to hand over the code before they are willing to pay, so basically I will be left at their mercy; if they don't pay, they will still have a working version of the system... So, is there any way I can inconspicuously code in some boo-boo's that are time related etc. Something that will bomb the mysql tables or break some code if it is not "unlocked" within a month etc. I'm not sure if people out tjere might have existing safeguard tools etc, so I'm open for suggestions. PS, I know about Zend's encrypter, but since it will live on their server, I don't think it will help much since they will need the decrypter on there anyway right? Thanks