Thank them for their input. Smile, and ignore it. Let me give you an example. Anne McCaffrey refused to listen to any suggestions from her fans about any of her series. Partly she seemed to fear possible litigation if she used their ideas. (This does ignore the fact that ideas aren't copyright and she could have used whatever ideas her fans gave her with impunity.) McCaffrey effectively pre-empted any fan feedback. This certainly would cover any fan wants for a series. Whether it covered fan critiques is another manner.
However, if a fan wants to criticize your series. Either again you can smile and thank them for their input or suddenly discover you need to be somewhere else in a hurry. Depart, with a smile and a heart full of relief. If it's written, either in print or online, just don't bother to read it. This will only hurt your feelings.
Take a leaf out of Isaac Asimov's book. The third of his three precepts for becoming a writer included "Develop a thick skin." Do thou likewise.
While it is nice have fans of your writing, there is no need to cave into their whims, desires, and their likes and dislikes. Although if some of their wants do lead to interesting possibilities, you are at liberty to develop them especially if they greatly enhance your series. Developing them thoroughly is a way to neutralize any claims you have "stolen" their ideas.