Deleting an item is very straightforward, but should be used sparingly. Once an item has been deleted you won't be able to recover it, so make sure you're only deleting items you've created by mistake or that really shouldn't be on the account.
The only user who can delete an item is the participant who owns the item in its current state, or an app owner. So even if your SMART Goal belongs to you, if you've sent it along to your line manager to approve only they will be able to delete this goal. Once it is sent back to you in a state that you own, you will then be able to delete it again.
To delete an item: