I agree with webmaven; one metaphor may not be enough for me either so here's my first:

For me, staying Clean in the relationship between client and facilitator is like operating and maintaining a fairly efficient one-way valve: information flows one way from client to facilitator* while as little as possible leaks back the other way - and it's not perfect.

Imagine two containers, one containing blue dye, one containing red dye. If the client is blue, the red of the facilitator becomes partly purple eventually, tinged with the blue client information. Meanwhile the more the facilitator stays clean, the more the blue client can remain free of red.

Perhaps not a valve then, more a flowing cleanish circulation with a wide conduit from the client to the facilitator and a narrow one the other way.

*though the facilitator receiving the client's information is an effect of the process and not the purpose