Deleting accounts is not recommended and can be dangerous if they include transfers to other accounts.
You should highlight the account in the side bar (list on left) and click ACCOUNT – EDIT ACCOUNT and tick “Inactive”, OK.
Now right click on the account in the side bar and select “Remove from side bar”.
Use the “+” to open a side bar edit window, add/remove accounts from side bar.
When you use the word “backup” are you using it the sense of Moneydance data set backups EXPORT BACKUPS or automatic backups set under PREFERENCES BACKUPS? If so, you can delete any unwanted backups of your data sets.
There must be an equivalent to a Windows right mouse click on Mac or many Moneydance features would be inaccessible.