Can I remove a participant from a document?

Of course you can! 

If you are the Creator or Editor of a document you can remove, demote or promote participants of the document. 

To do this, find the sharing settings by clicking on Share in the upper right corner of the document. This will bring up a modal where you can view the entire list of participants. Click on the role, to the right of the name, to change their document role. 

Note: If your document has link sharing turned ON, anyone that still has access to the document link can re-join the document after being removed.