Well, before you withhold all the food and water, do you know for sure, if you have any "mothers" or pregnant cats in your yard each day? I guess that is the only thing that would keep me from taking away all water and food immediately, because if they are expecting kittens, and you have been feeding them for more than a month or so, it would seem inhumane to all of a sudden stop. I know, I know, I said to lift all water and food, but I have also given a link to a website that you may want to look up your zip code and see if there are any possibilities of getting help from anyone in your local area.
The only thing that makes me feel rather badly, is that last year when this all started all over again for me (yes, I knew better from nine years prior, but my heart won out and not my head), I started feeding the two cats in my yard in the snow. It started because there were POOR birdies out there....hahaha... and I threw out old cereal I had for the birds, and the two ginger cats showed up. UGH.... Well, anyway, another cat, a tortie (multicolored) cat showed up after they did. I still kept feeding them, thinking I would find help to have them speutered (spay/neutered), and thus, my dilemma started..... I did find a woman to help me, and I trapped the tortie, who was pregnant by now, and had four little tiny kittens running along with her. So now I was really in a fix. I took her to this woman who was going to take her and the kittens and have them fixed and put up for adoption at a Petsmart. Well, her vet had the female mother asleep on the table, and tested her for FIV which is feline immunodeficiency virus, and lo and behold she tested positive! Well, the woman helping me, called me to tell me she would NOT PAY for this cat to be spayed and wanted me to give MY permission to have the vet put the cat to sleep. I had no other choice at that moment and I have lived with this guilt ever since that I could have brought that mother cat back, to my yard, to be with her eight week old kittens.
The story goes on, and on, and if you wish me to continue, I can, but that is why I feel that once you start feeding and giving water to strays or ferals, you have to know that you have to make a choice one way OR the other. I know now that I will NOT start to feed or water strays or ferals again, because I am not in a financial or physical position to do the right thing for the cats.
So, my advice to you is if you are truly feeling guilty over lifting the food and water, then please check out the website below, and make your decision on what you really in your heart want to do. OR WHAT YOU CAN DO...... If you are not able to keep up with this, then it is best to STOP now. Call for help, if you will feel better about it, but do not keep feeding them after you have no more in your area to feed.
Gradually will not work, as the cats just think that you maybe forgot to bring out their daily rations. You truly must decide what is best for you, and also for the cats. They CAN find other means of survival, though it may not be the best way, but don't beat yourself up over it. I have been there too many times now, and my health has really taken a toll. I love the cats so much and they are like family to me, and I keep my own, inside ONLY.
Let me know if you need any further help, and I will help if I can.
God bless and good luck!