Ummm… I believe you are mistaken. POP3 does not support folders, so any spam folder you have on a device which is using POP3 is a local folder, not the same as the folder on the server. It might just have the same name. (Unless I am SERIOUSLY mistaken.)
You can’t, unless you switch to IMAP.
Some people tend to say this, but when looking into the details it is usually based on misunderstandings about the capabilities of the IMAP protocol and IMAP clients.