For the spam folder, you can delete all messages without having to "select / delete / repeat" by:
long pressing it (the folder, on the app's main screen) and choose "delete all"
or by opening the folder's message list and doing Menu (three dots) and "delete all"
It should also be possible to do this easily in web mail (without the messages loading into AquaMail at all), which is going to be faster than using any mail app.