IMO, the best strategy is to develop a very rapid & robust "cluelessness filter". I don't get many requests to pick my brain via email but it does happen a lot

IMO, the best strategy is to develop a very rapid & robust "cluelessness filter". I don't get many requests to pick my brain via email but it does happen a lot face to face. At networking events, the first 30 seconds of my interaction with someone is essentially focused on testing for cluelessness. If it's clearly worthless to have a conversation with them, I'll disengage and politely decline further requests to meet up. If there's even a spark of something interesting, I'll generally meet the again for coffee and the half hour of one-on-one time determines whether they're worth regularly touching base with. I don't know how to do it via email, maybe ask them to set an agenda for the meeting before you agree to do it and use their reply to that as a filter. I've gone to enough startup events that I really don't feel guilty anymore outright snubbing the clueless. There's a virtually 100% certainty that they won't amount to much of anything so why waste your mental energy on them.