Currently large spam boxes in Front are impossible to handle. Front has taken the view that users should not be allowed to mass-delete spam and must select each message (not ctrl-A) but for a spam box with 1000s of messages that isn't practical.
This could be solved by any of:
Allow ctrl-A delete. Your users are grown-ups. Allow them to do what they want. All other email clients allow this that I know of
When an email has been deleted on the server, delete it in Front. Then users could delete the spam on gmail and not still see it in Front
Allow delete after time limit. Office365 and gmail both allow automatic deletion of spam after a delay. If it was labelled spam and I didn't search for it in 3 months then this was not an important message
If emails do get accidentally deleted after being spam-classified, the worst case scenario is that the email has to be sent again. It's fine to let your users do this.