What is good amateur practice if propagation changes during a contact creating interference from other stations using the frequency?

High-frequency (HF) propagation is dynamic, and changing ionospheric conditions can suddenly bring previously unheard, distant stations onto your frequency. Because no station has priority access to a frequency (except during emergencies), you cannot simply demand that the other station leave. Good amateur practice dictates that you politely contact the other stations and work out a mutual solution. This might involve one or both groups shifting frequency slightly or moving to a completely clear frequency. Decreasing power won't solve the interference you are receiving, and switching sidebands would make your own signal unintelligible to your contact. Communication and cooperation are the only effective solutions.