I do feel that Nabble's Captcha facility does need upgrading. It should allow the visually impaired to interact fully with a forum. However, as an administrator, you can remove the captcha facility yourself by editing the NAML code behind your forum.
Go to the "Edit this page" link, search for the macro "captcha_control" and remove the call to that macro from the macros where it appears. (A quick glance suggests that is only two places.)
Pedro, one of the Nabble team, should be able confirm this and give further advice if you need it.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.