I have the same exact issue. Last pass was no help.
In truth, there's no way to 'hide' a shared password once it is entered on a website, as LastPass has no control over the site's security functions.
However, if you disable theAllow Recipient to View Passwordsetting, LastPass itself will not show the credentials to the user in-app.
Understandably, this is not the most secure system due to the on-site limitations, so please keep this in mind: Don't share any credentials with anyone you do not inherently trust yourself.