1. Copy those folders to another location which isn't synced with Dropbox.
2. Use selective sync to unsync the original folders (Dropbox will delete them).
3. Copy the backed up folders to the original location again (They will remain unsynced.)

Source: https://www.dropboxwiki.com/tips-and-tricks/exclude-folders-from-syncing

1. Copy those folders to another location which isn't synced with Dropbox.
2. Use selective sync to unsync the original folders (Dropbox will delete them).
3. Copy the backed up folders to the original location again (They will remain unsynced.)

Source: https://www.dropboxwiki.com/tips-and-tricks/exclude-folders-from-syncing

PS: I agree with the comments below. There seems to have been some change in behaviour in Dropbox's sync function which creates selective sync conflicts. If you are facing this issue, I found yet another alternative - I moved my folders containing large files out of Dropbox and rather put their links (shortcuts) in the desired locations. This syncs them as files and not folders.