On the website, go into the level of the course, then click on the Ignore button on the right:
Tick the items you want to ignore, then click Save at the bottom.
You can select All at the top to skip the entire level:
Alternatively, for the app, you can simply choose which level to learn/review from the main dashboard.
On the website, you can also ignore words during a learning/reviewing session by clicking the ignore icon:
To sync the app, pull down on the Home tab screen to refresh your account. Any ignored items will be indicated in the levels: