Dealing with an over-enthusiastic co-worker

I have a co-worker in my department who means well but that can be difficult to deal with. They are CONSTANTLY pitching ideas. Sometimes, the ideas are great, which I will always acknowledge when they are, especially when they are successfully implemented. However, there are oftentimes when these ideas aren’t realistic and don’t really fit in with my needs. I always try to gently tell them that it’s a good idea, before giving the reason why it’s not feasible at the moment. But because they are always coming up with ideas, I feel like I am constantly turning them down. Their implementable ideas often get overshadowed because of how much they are always trying to come up with ideas. Every time they have an idea that I nicely turn down, the dynamic of the conversation completely shifts because they take the rejection so personally. My manager has also dealt with this issue and has also struggled to come up with a solution other than what we’ve been doing . It is exhausting and frustrating to constantly be the “I like your idea…but” person. Their enthusiasm is great, but it just needs to be channeled more appropriately. This is mostly just to vent, but any advice on how to better approach this situation would be greatly appreciated.