is there a way to clear customers' cart content in netsuite

From the backoffice? nope.

there's a removeAllLines method if you want to do it from a website

or www.mysite.com/app/site/backend/emptycart.nl

i would apply that method

I'm not 100% sure what you are wanting to do, but this explains a way to set up an empty cart button on a site and used the method I mentioned above: https://developers.suitecommerce.com/add-a-button-to-remove-all-items-in-the-cart

well for some reason whatever is in cart right now is throwing an error - and i know for a fact it has to do with an item thats in cart. so i need to remove that item from the cart to be able to add more

The easiest thing for the customer would be to send them to the /app/site/backend/emptycart.nl endpoint as Pablo mentions above. It's legacy from our WSDK/SiteBuilder products but still triggers the cart to empty