Try looking for previous posts on a subject first you might find the question has already been answered.
Firstly ,you could keep them, having food in the red is not always a major problem.
Secondly, you could attack someone, you may get points for doing so if they are a weaker opponent, but its a sure fire way to get rid of them and could help someone from your own clan.
Thirdly you could just dismiss them .Go to town, open shrine of Odin, look at troops in town, Click on troop picture ,similar to training them,decide how many to dismiss.
There are a number of boosts that can increase food like, Hero items , VIP , and yielding ,without building more farms or increasing farm level, which is easiest way to increase stock
Now that's a mystery.
Unfortunately though I saw your post and was replying to it , my longer answer was not shown till after the person who suggested you dismiss the troops.though you may have decided on this solution anyway.
I have never dismissed troops,so have not come across this situation, but it seems at odds with expectations.
Are you sure they are the same troops that you dismissed and not some lost in a recent battle?
Maybe they do go to the infirmary, where exactly are they ?in healing or resurrection? if they are in resurrection and you do not revive them, they should disappear after a week.
You don't have anything to worry about wherever they are,unless they are still consuming food ,which I've seen suggested they don't whilst there, though I am not sure if this is bogus.
You might supply the answer.
One last thought, which may prove helpful to a solution. Count how many troops are in the infirmary, choose to dismiss another, and see if the count in the infirmary changes .
The mystery deepens.
The infirmary always has that floating pink cross.
As I have not done this before,I have just sacrificed one of my free troops to see what happens. It did not go into the infirmary, it disappeared completely as I had expected it would.
It seems you have a glitch, but you possibly have the chance to change your mind at some stage and keep the troops you wanted to dismiss.
They appear not to be consuming food ,so you have eliminated that problem, however they are taking up room in the infirmary which you might need later.
If they are in the healing section they will stay there till you heal them, if they are in resurrection, they should disappear within a week