While Apple doesn't officially want you to be able to choose which files iCloud keeps locally and which it doesn't, there are currently two ways of getting around that restriction (kind of, at least):
(1) Via a Terminal command. The command
brctl evict name (with
name being replaced with the path of the file or location (assuming it's something within your iCloud Drive, of course)) will remove the local copy of that file but keep storing it in iCloud Drive, available to re-download at anytime (with the little download-arrow next to the file in Finder). This also works for entire locations, i.e. if you enter the path of a folder, it will recursively remove the local copies of all files inside (but keep them in iCloud). This does require the "Optimize Storage" option to be turned on.
Now there are some downsides to this, however. Firstly, it doesn't
always seem to work
forever – while it will always instantly remove a file for me, some sturdy files would just randomly decide to download again a short while later, which is especially annoying with large files that are gigabytes in size and are suddenly hugging your internet connection. This is most likely because iCloud's intelligent algorithm for optimizing storage will take over sometime after you've entered that command and will decide that the file was used so recently that it should be stored locally. Usually it ends up working after sometimes entering the command multiple times, but it's still not perfect.
The other "issue" is (though for most people it won't be an issue) that this also seems to delete most of the metadata of a file, which (I believe) won't be deleted if iCloud's intelligent "optimize storage"-algorithm removes the file on its own. This means that Finder previews might just show a blank file page and that Spotlight won't be able to index the content of these files. Like I said, for most people this probably isn't a big deal, but it's worth mentioning.
(2) The second possibility is via a neat little free Finder extension called
iCloud Control. Now to be clear, this extension doesn't do anything different than the Terminal command, all it does is basically offering the evict-command as a Toolbar command in Finder so you don't have to go through the Terminal-path each time. This means that all the up- and downsides from the above paragraphs will carry over to this tool – sometimes, files will re-download after a while against your will, etc. Still, it's a huge convenience to have this right within Finder itself, and if removing the local copies of files in your iCloud Drive is something you see yourself doing on a more frequent basis, then I highly recommend this tool.
In addition, @OP you should be aware that if the "Optimize Storage"-option is turned on, iCloud will
automatically remove the local copies of files on your drive if you run out of space. If you open the info-window for your main drive in Finder, you can actually see there how much space is available if Apple were to remove all of the iCloud-stored files under "free storage": the "purgeable" storage in this section will show you the amount of storage that is currently occupied by the local copies of iCloud Drive files but which can and will be deleted if you run out of space.
In conclusion, I really hope that Apple adds a manual toggle for removing the local copies of files to Finder sometime in the future, and the lack thereof is one of the few downsides of Mojave for me. The functionality is clearly already there, as indicated by the existence of the Terminal command (though Apple would need to work a bit on having user commands take priority over iCloud's own local storage management), it shouldn't be too hard to add a "Remove local copy of file"-toggle to the right-click menu in Finder. iTunes and the Music app on iOS already have the option to download and remove the local copy of music at will, and so does iBooks on iOS. There is really no reason for iCloud Drive in Finder not to have it.
