To close a project click on the more button in project toolbar and select the option "Close Project" from the menu, as highlighted in the screen shot above. This closes or marks as 100% complete the project and all tasks inside this project. The impact of this is:

• No-one can book time or expenses to this project or its tasks after today
• The project and its tasks are marked as fully completed or closed
• Unbilled / billable time, expenses or fixed price deliverables booked to this project will still get billed in the next billing run just as they do for any completed task
• Financial metrics will not get updated again after this (apart from new customer invoices)
• The project is not deleted. It will still appear in reports and the project search screen and its tasks will still appear in my tasks if the user selects to show closed tasks

There is an enhancement coming to not show closed projects in the project list unless "Show closed" checkbox is set.