Part of the job is working out who to invite into your office and who to refuse, and doing so in a way which doesn't offend or close off avenues which you may need to explore in future.
Normally I simply explain that I prefer to meet if, and only if, I have a specific product requirement at the time. I often give the potential visitor something to go away and research - like, for example, what switches can you offer that fit an envelope X x Y x Z mm and are rated to 100,000 actuations or more - and make it clear that the objective of the meeting is to discuss that specific product.
You still get the waffle, of course, but sometimes it can be helpful, especially if they know enough about your business to identify what's likely to be useful and what isn't. And at least you've spent a significant proportion of the time on the actual problem you need to solve there and then, which won't be the case if you've not allowed your visitor to come prepared.