I try and stay out of Diplomacy, as I don't want to get involved like I did in the past, I play more casual now with little regard for strat (waiting on Rome 2) but can assure you Hospitaller means no disrespect, Its just hard for them when FCC keeps getting reinforcements from your direction. I understand letting everyone use your lands, as you let Hospitaller for a time being, their issue is you are at our borders making it an easy staging ground for FCC.
FCC could make an army, add trading goods to head your way for a swap, then use that army to reinforce in our lands claiming they were just their to trade and the secondary action was not planned.
Regardless, You did make it clear and gave them fair warning. It just puts them in a very awkward position. I would have thought your best stance would have been to deny all military presence what so ever, meaning heading towards to attack a near by faction, reinforcing staging area or any army meant in a hostile manner. I understand the need for FCC traders to come and openly trade in your area and respect you lads for your neutrality, but allowing armies that then head into your neighbors land to attack or reinforce should be looked at the same way as armies coming in to your lands to attack those said armies, either should be denied.