You will need to change your CAPTCHA. the one you are using was broken by SPAMbots 20 odd years ago.
Please visit
Preventing Spam in phpBB3 for various options for fighting spam in phpBB.
... or use the inbuilt Q&A CAPTCHA
How to configure Q&A
- Administration Control Panel (ACP) > Spambot countermeasures> Installed plugins > click the dropdown box and select Q&A (it will be greyed out)
- Click the configure button then click add
- Add your question and answer (you need a Q&A for each language you have installed) click submit
- Click "back to previous page" then click "back"
- Select Q&A from the dropdown box (again)
- Click submit at the bottom of the page.
- Done.