Standing into my tests:
i have an user
paco that exist into phpBB, but not in WP, and when i try to register an user with the same email in WP, using the WP members page/form this happen:
if the user is active in phpBB
it is added into WP at the time the user click into the registration submit, and the WP member warning say:
Sorry, that email address already has an account.
Please try another.
then i delete again paco and as deactivated phpBB user, i try to register him again and the result using the WP member registration form is:
the page redirect to the default WP login page, and the warning from the integration plugin display:
Notice: the username is currently inactive into our forum. Please contact an administrator.
and the user is NOT added into WP because not active into phpBB.
I did not try it if the WP default login page would be not available because disabled by some plugin, in this case the redirect should go where it is set (i assume) by the plugin that disable the login page, maybe.