items deleted keep coming back
I think Zotero is a great product and I'd like to keep using it but I've had too much frustration in the last several weeks over sync problems. After spending weeks staring at endlessly spinning sync arrows, and, on the rare occasion when syncing worked, going through lots of conflict resolutions, only to have duplicates and items I want to delete put back into my collection, I decided to suspend autosyncing, create a clean copy of my library on one computer, and upload that version to the server. I spent hours deleting all the pdfs and duplicates from my library, then selected the reset option "restore to zotero server" but the server came back to me with over 600 conflicts. I selected the local version of each one, and then the server downloaded my library back to me - with all the pdfs I had tried to delete back in! Either I did something wrong, or your sync is not working right.
We've clarified this in the latest dev builds.
Unless you're referring to group items? Those aren't affected by the reset options.
Deleted items still shouldn't come back. Don't use the reset options. Just delete what you want, with auto-sync enabled, and the changes should propagate to the server. You can check it at any point on the website.
And if you want to make sure your work is preserved at any point, make a proper backup of your Zotero database. Then, in a pinch, you could always delete group library items from another computer, check the website to make sure they were gone, and then perform a Restore to Zotero Server—that doesn't clear groups, but it does force the Zotero client to upload everything it has.
(If the sync that undid your changes occurred within the last 12 hours, you could even disable auto-sync, revert to your last automatic backup, verify that has what you want, clear the group items from another computer and make sure they're gone on the websites, and perform a Restore to Zotero Server on the computer with the latest data. You shouldn't really need to do any of this, though.)