Intall a captcha generator is a good idea but you need to provide us with further information, basically what your forum software is to see if there is already a plugin or mod that helps you with this task.
In the mean time, simply disable the instant registration feature adding a verificaction option, either clicking on a link to activate the account, or manually approving new accounts as admin.
Are there any good captache genrators ro prevent robo sign-ups?
