Warning this does not delete the portfolio item, it just removes it from a block so you can add it again later.
To remove the portfolio item from an article or gallery block:
Click the edit button circle that appears in the bottom left corner of your browser screen when you are logged in and viewing your portfolio.
Move your cursor over the portfolio item on the "Article Block" on your page. Controls should appear on the item, including a delete icon.
You will need to click it again to confirm deletion.