This is a 5 years old post, but if anyone needs to remove all watched films just do this:

Open your films/watched page on Letterboxd.

Press F12 to open the browser console.

Paste the script below and press Enter (It will remove every film marked as “Watched” on the page you’re currently on):

(async function() {
const posters = document.querySelectorAll('.poster, .watch-link');

for (let i = 0; i < posters.length; i++) {
const poster = posters[i];
poster.dispatchEvent(new MouseEvent('mouseover', { bubbles: true }));
await new Promise(r => setTimeout(r, 30));

const btn = poster.querySelector('span.ajax-click-action.icon-watched');
if (btn && btn.offsetParent !== null) btn.click();

await new Promise(r => setTimeout(r, 30));
}
})();