Hi Pokie!
Well that's exactly what I've done before posting here, but that is more like a trick, I tought that there would be a more "technical" way to do it. Anyway two of those fleets were still moving if someone would've take command of them, and they were very close to the land mass, after that I brought them even closer, and yes they didn't move anymore, that's why I was looking for a definite way to disable them from moving.
However, if there is no other way that works for me, as I actually use the fleets to guard some islands, so I placed them really close of the land mass and therefore they cannot move, none of them moves anymore.
Thank you for the thought anyway!!