A method can include closing a valve interconnected between sections of a washpipe of a service tool string by displacing the service tool string relative to a completion string, thereby preventing flow through the washpipe. A well system can include a service tool string reciprocably received in a completion string, the service tool string including a washpipe received in a well screen, and a valve which selectively permits and prevents flow through the washpipe. The completion string can include a structure which operates the valve in response to displacement of the service tool string relative to the completion string. Another method can include closing a valve by partially withdrawing a service tool string from a completion string, thereby preventing flow through a washpipe of the service tool string.