Complaining is natural. For some people I know, it is virtually an art from. And complaining clients is not something that is going to stop a good solutions-focused practitioner in their tracks.
Ranting and venting of steam can have their place within an SF conversation. It's OK for people to have feelings (of any type) and to express them. As a webinar participant put it neatly the other day, ‘we all experience the Present Imperfect' .
People complaining are mostly going into detail about aspects of their issue that they don't want. If you listen carefully, you can find or deduce a strand in there too about what they do want - and which they are willing to address.
Our task as coaches, therapists or consultants is to discover that strand and amplify that part of the conversation.