Don't use the the checkbox to hide the two login items. Select 'ClamXav Sentry', press the minus sign next to the plus to delete it. Repeat it with 'WDDriveUtilityHelper'. You just hide login items that should work in background, not login items that aren't existing anymore and can't get shown in the Finder by right clicking on them.
Usually there are some plist files related to the login items in those two folders:
/Library/LaunchAgents
/Library/LaunchDaemons
I suggest that you first make a backup of those ClamXav and WD plist files there, instead of deleting them directly.
There are many uninstaller or cleaner apps in the Mac App Store. Some are offered for free from time to time. Beware that most of these tools do have the potential forcing you to throw away files you'll still need, if you don't use them with caution. If you're planning to make intensive use of such apps, always have a recent system backup ready. From my experience, most of those extra maintaining tools aren't necessary, especially if there is no real problem around. There are some good tools for the Mac, but in general macOS should be almost maintenance-free.