Note: use these Tutorial steps at your own risk. The safest way to uninstall a Pack is to uninstall The Sims 4 entirely and then install it again.
It seems like after you download and install a Pack for The Sims 4 in Origin that you just can’t remove it at all since the ”Uninstall” button has not been provided for Packs.
There is an easy solution for this though!
To Uninstall a Pack, just find a folder where all your Origin Games are installed, open The Sims 4 folder and delete a Pack folder of your choice.
Pack Folders: (will be updated as new Packs release)
I’ve opened The Sims 4 after deleting a Pack just to see if things are running smooth without any errors or crashes – and it did!
Note: Save Games that contain uninstalled content will have their stuff from Sims and Houses removed.