the option to prevent the users to create a blog is checked, but i think that "signup_blog()" does not take into account that parameter
I cannot reproduce this. I installed on a fresh instance of WP, set up my network to ONLY permit users to create accounts, and the signup page is not only just for accounts, but it keeps the user on the page, on that site. Works exactly as expected (with an empty param warning I'll fix).
You'll need to explain, in clearer detail, what your setup is on the network (what registration option did you pick), and what ones you picked on the site, if you want me to debug this fully. Right now, using the plugin as I intended, it does what I intended, so I can only guess you're doing something unexpected :) That's not BAD, but I want to make sure it's within the scope of what I want to support and maintain.
What I did:
1) Network settings - Users can register
2) Network activated the plugin
3) Per site: manual registration, checked 'per site' and picked my page 'join.'
4) Put the shortcode in join
Then as a logged out user, I went to join and it correctly showed me just what I should see: http://cl.ly/image/3z0M1s3x2O3i
Also, the button to register is labelled as "Register" and not "Sign Up" in the wordpress registration page
That brings up exactly why I use helfjmm.
The reason I'm using helfjmm is that if WP changed and I wasn't, everything would break. On the other hand, if I use my own and people submit language packs, I'll add them and no one will break. I'm not going to change it back right now, it works the way I want it to. If you want to submit a language pack, I'll add it and credit you :)
In the next version of WP it will be Signup (one word).
I'm fixing the param warning, and cleaning the translation. Submissions always welcome.