There is not a function to remove all users attachments, but there are some tools to help you manage them.
Admin CP -> Attachments -> Search
Use the "Find All Attachments Older Than X Days" option. The following page will let you review and prune the matching records.
See this thread it might be useful.
https://vborg.vbsupport.ru/showthread.php?t=296552
Why do you need to delete all attachments?