OMG this "My Mac" sync thing was really doing my head in but I eventually got it to stop trying to sync that folder. Here's what I think I did:
Disable the Backup crap as outlined above. That wasn't enough though .... it was still sync'ing away. Wtf !!!
Go back into the [Preferences > Sync > Choose Folder] and deselect "My Mac" there. That wasn't enough though .... it was still sync'ing away. Wtf !!!
Open the Dropbox descktop app and manually delete the "My Mac". Then is stopped syncing. Hurrah !!!
I have to say ... the Dropbox user interface has totally gone to sh1t. It's just all over the place now. A maze of stupid upsell options and other crap that I don't care about ... multiple entry points OMG. It's just totally confusing to me now. I just want to select a few folders that I want sync'd to Dropbox. Please make that simple use case easy. I don't freaking want Dropbox auto-syncing random fodlers for me or auto-enabling Backups - I'll look after that myself thanks !!! Or at least make it easy to turn off that sh1t !!!
I tried your steps and they worked perfectly, thank you!! I freaked out because I couldn't find anything that used to be on my desktop after that I backup up my Mac overnight (and support was closed for the weekend, nice). Also, I found that I had to add one more step in order to find my desktop items again because they were originally set up to iCloud: Mac System Prefs > iCloud > Options > check "Desktop and Documents Folders". Mini heart attack averted. Seriously Dropbox wtf. I'm sure there's a way to do this so you don't lose your files like I did, but it is waaaaay too confusing, especially because it's presented that it will be simple. Old fashioned way every few months onto external drive is the most failsafe.
My Mac has also loaded all my computer files to Dropbox when I just wanted to upload one file. I tried to stop it by deleting the files on Dropbox and now may have deleted them on my computer as well as I'm missing a lot of files. ARGH! Hoping they are backed up in the cloud.
I tried to follow the advice above about opening preferences and then doing some steps. I can't find the system bar or the preferences tab or whatever so I can't follow the steps to delete the MyMac I've clicked on everything. Please help. My Dropbox home page and other pages I click on doesn't have the menus you're referencing.