Only the project owner can remove collaborators from a project.

1. From the list view in the "Collaborators"-window, click on the name of the collaborator you want to remove. 
2. On the "Information" tab in the "Collaborator details" card for the collaborator, you find the "Remove user from the project"- option at the bottom left of the card.
3. Confirm you want to remove the collaborator by clicking "Remove".