How to close thread in SugarCrm Forum 

Press the 'Mark Correct' button and it is basically closed. It'll be viewable so others who have the same problem can see the answer.

You can confirm by marking Alan Apter's response as correct   

Edit: Or do you mean to remove/archive a discussion?

If you are looking for other questions regarding the community itself I can also recommend the Getting Started  space where you find a lot of details about the functionality in the community like difference between questions and discussions and others. 

Best regards,

Dennis