In your desktop Lightroom, right-click on the collections and choose to stop sharing. That will remove them from Lightroom Mobile.
Thank you very much, JimHess.
Below, you can see my homepage as I enter the LightRoom suite online.
I didn't create collection but I uploaded many pictures (you can see them in the second box on the right).
I can't find any button to delete them.
And there is no option to delete the account as well.
Jim explained, above, about removing photos that you've synced from your desktop. You disable sharing in the LR Desktop app, not on the website. Are there other photos on LR Mobile online that you want to remove that are being shown in LR Mobile online that come from another source?
Lightroom Mobile is part of your LR CC subscription and you cannot get rid of it, just remove the pictures you've shared.
Are you saying you want to cancel your CC subscription and stop using LR altogether?
Additionally, LR Mobile remembers any photos you've worked with on your device and synched back over to your desktop.
To remove those you can use the website. Click the Organize button and select them, then click the Delete/Trash icon and this will remove them from online display but leave them in your LR Desktop catalog. When selecting many photos from the website, it does work to click on the first then shift-click on the last photo and all intervening photos will be selected.
Thank you very much, ssprengel: I made it to delete my pictures!
Yes, I confirm you I'd like to cancel my CC subscription and stop using LR altogether.
How can I do?
If you want to cancel your CC plan, you can contact Adobe as instructed, here. There will likely be a cancellation fee because your subscription was an annual plan and you've only paid part of it:
Go to the Adobe website, login to your account, Click the Manage Account link, You'll need to Sign in again and or re-enter your password, Click Manage Plan, Click the Cancel Plan link. From there you are on your own as I won't go any further now that Adobe has partially straightened out the mess they created with the .2.x update.
if you go to the Lightroom mobile tab in Lightroom preferences, there is a button there that will enable you to delete all data.