I am reading my Mail from many different E-Mail-Accounts on my Windows 8 Phone. Some of them use "plain" IMAP from simple hosters, so there is no server-side Spam-Filter-support whatsoever.
Is there a way to filter the spam from these accounts?
Not on the phone itself. There is no junk control on the phone. You could forward them to an outlook.com account and have it do the spam filtering for you?