Understanding Front's spam feature
Front sits on top of your DNS provider, so Front will display all the messages that make it past your email provider. This is the case for emails that are forwarded to Front (non-Gmail/Office 365 accounts). Any email that your provider marks as spam will not appear in Front. With Gmail and Office 365 inboxes, Front will pull messages from Junk and Trash, so spam will appear in Front.
Front offers the ability to mark a message as spam. Find Mark as Spam by clicking on the three dots to the right of the conversation toolbar.
If you're using Front with Gmail, marking a message as spam moves the message to trash and blacklists the sender. When a sender is blacklisted, any future messages from them will go straight into your Front trash.
This effect is global, across your whole Front team, so please don't mark any messages as spam from senders that your teammates might care about. Instead, you can use a rule to keep those messages out of your way.
If you are using Front with Gmail, then Gmail's spam filtering will be applied prior to your emails coming into Front, so this shouldn't be necessary too often. When Gmail marks a message as spam, those messages will flow directly to your Front trash.
Mark as Spam vs Block Sender
If you're connected to Front via Gmail, Mark as Spam will tell Gmail that this message is spam. If you're not connected via Gmail, then it will only trash the message. When you Block sender, every subsequent message from that sender will be trashed automatically.