You can absolutely block individual users on connect. On one of the bot's posts, hit the ellipses and select: block >block user

Also, you can check the currently blocked accounts here:
https://sh.itjust.works/settings
(Click the blocks tab) You can add add users from there, although I usually do it from their profile page.

I don't know about connect, but you can block an account in your settings in the web UI easily enough. That should then also be blocked whenever using whatever app.
Just hit the block button from that user profile.

After blocking is it still visible on the webinterface?

And usernames need an @ before them to distinguish them from an email address.

@bOt@zerobytes.monster