you can use the class-extension within the package to achive that:
(class hn_captcha_X1 extends hn_captcha)
* This class is an extension for hn_captcha-class. It adds a garbage-collector!
* Normally all used images will be deleted automatically. But everytime a user
* doesn't finish a request one image stays as garbage in tempfolder.
* With this extension you can collect & trash this.
* You can specify:
* - when the garbage-collector should run, (default = after 100 calls)
* - the maxlifetime for images, (default is 600, = 10 minutes)
* - a filename-prefix for the captcha-images (default = 'hn_captcha_')
* - absolute filename for a textfile which stores the current counter-value
* (default is $tempfolder.'hn_captcha_counter.txt')
... (read on in the classfile itself)