Add way to delete spam/unused users

### Describe the problem

My weblate instance has been targeted by a large number of spammy sign ups. Automated bots create accounts to link to their websites. Due to `rel=ugc`, this won't actually help their SEO. But it may be resulting in my website being seen as less trustworthy (I've been banned from Bing recently for some unknown reason), and having a large number of fake users is annoying

### Describe the solution you'd like

There should be a way to delete spam/unused users. The criteria could be not a member of a project, never contributed to the project, and not used their account within a set period of recency

### Describe alternatives you've considered

It would also be good to have better checks to prevent spammy sign ups.

Or perhaps a way to disable user profiles completely

### Screenshots

_No response_

### Additional context

My instance is at https://weblate.rubenwardy.com/