I've been using this to great benefit also - require the person to put a decent amount of effort into the idea first. Some people resent you for it, but those that are destined to succeed and are able to make it will keep persevering.
I used to not do this and wasted a lot of time in the process. Do you find it hard to shake the reputation you had from before? I know I do.
Yeah it's definitely hard but what helps is to really be upfront and candid about how limited my time is from the beginning.
So if someone refers someone to me, I'll tell them that I'm happy to assist however I can at this point but that my time is limited for the foreseeable future. You can do that directly or just by letting them know what you're currently working on. I want to keep the opportunity open while being honest about how I might be able to assist. Down the road it may even make sense for me to shift my time from one of my other projects/companies to the new opportunity that was cultivated from the reputation.
If it's an area that I'm not interested in, I'll say that too - "the restaurant (tech) business is tough and I don't have expertise in that arena so it's not for me". But I'll usually provide some strategies and let them know that the dialogue is always open with me.
I used to not do this and wasted a lot of time in the process. Do you find it hard to shake the reputation you had from before? I know I do.