I’ve just noticed that if users hide their profile, the buttons to ignore them or send them private messages are also hidden. There are workarounds though to achieve the same, so maybe others will find this useful as well:
go to your own profile and
to ignore / silence a user: select Preferences → Notifications → Users
to send a pm: select Messages → New Message
where you can enter the desired user. A bit more cumbersome, but at least it works