I feel your pain and have been in your shoes. It can be an overwhelming situation. It is difficult to know what to say. ( flannel makes wonderful, warm quilts)
One way I managed to slow down the ( gifts) was to make comments while with some of the guilty parties like....'I have so much fabric, patterns stacking up it will take my grandchildrens lives to use it all' this often opened the door to conversation that helped them understand. I also started asking for gift cards, so I could pick up what I needed/ wanted. As for my spouse, I took him to the quilt shop with me, ( he had ventured into a store on his own & spent almost $400 on a whole collection- which is still 7 years later in the bag on a shelf waiting for me to figure out what to do with it)
When I took him with me I didn't let him ' go crazy' he picked up patterns which I said- ' I will never make that' he pulled fabrics, which I said --',I will never use that'
Now, at Christmas he is great at stuffing my stocking with new rotory blades, post its, needle threaders, rulers/ gadgets recommended by the ladies at the quilt shop who know me.
It took time & patience but I did manage to get all day the ( well meaning friends & family members) to stop dropping off bags of fabric I didn't want. I also started showing them my space, my projects, sharing the process a little which helped them see they had no idea what I would want/ need. Its all a patient process of sharing, open conversation. And those patterns you have no interest in.... Put them on the book shelves & forget them for now, years from now you may pull the pattern out & decide you do want to make it. My ( Unusual Lone Star) quilt which I have shared pics of here on the board is such a pattern. My sister bought it years ago ( over 10) because it was paper pieced & she knew I had done something paper pieced. I thought.... That is the ugliest quilt I've ever seen! I will never make that ( and the pattern was way more complex- out of my comfort zone....so it was stuck away. All these years later I happened to come across it while looking for something else and voila... I made it! Great quilt! With quilting you just never know what direction you will go as time goes by. Interest changes, techniques grow, catch your eye, 5 years from now you may find yourself loving applique. All that flannel may turn into great cozy, warm, snuggly raggy quilts ( you could discourage some of those by making them raggy quilts with the flannels they bought) or donate them to a local place ( emergency shelter, senior living facility, hospice, american legion, Knights of Columbus, any organization that puts together packages for people in need. Good luck. It took me a couple years.