I used Lazyexpunge.

I tried to create .DELETED without any hints and see that the creation was successful.

I want to prevent users from creating .DELETED, what should I do?

My settings

plugin {
lazy_expunge = .DELETED/
}
#

namespace {
prefix = .DELETED/
hidden = yes
list = no
separator = /
location = maildir:/var/vmail/backup/expunged/%d/%n
}