Enable preventing e-mails from registering

### Describe the problem

Some accounts are having spam links in their profiles, others contribute with malevolent translations. Block is not enough.

### Describe the solution you would like

Spam users should not only be deleted, which is currently possible, but also prevented from signing up again with the same address. We should hash the blacklisted addresses to keep this privacy-friendly.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_