Joeska, you may want to install the latest version of your forum. It appears a number of anti-spam and anti-bot measures came with version 3.0.6. The most successful method of deterring bots on older installs is a custom-validation drop-down or custom profile field selection. I'm not sure on the functionality but an upgrade and enabling the CAPTCHA or Q+A feature may be your best route here.
https://www.phpbb.com/customise/db/mod/sortables_captcha_plugin/
There is also this one that is built into phpBB 3.0.6 or greater.
https://www.phpbb.com/kb/article/how-to-configure-q+a-captcha/