um... depends on what you are trying to stop.
time delay is irrelevant. so would any deals that involve transaction fees (some sort of npc gold sink.. eg 1 gold per 100 bushels traded), if the idea is to support another realm (in the medium/long term) by giving them funds and not caring much about the cost.
it'll be tedious, but just imagine 2 cities with huge food reserves (and overall realm not in huge shortage). they stick up a ton of trades.. a week later (really slow trade, for example), bonds shuffled around. food shuffled around with minimum rot to boot
if you somehow manage to close back and forth trade between 2 regions, it can be done with 3 or 4. (realm A rurals sells cheap food to B cities, B rurals sell expensive food to A cities)
it's not too different between some noble carrying a ton of gold around to give to another realm via bank or guilds/religions with the advantage of not getting robbed (unless trades gets robbed or some such on the odd occasion)
it's just a matter of finding balance between risk of getting robbed / cost doing the business and not pissing everyone off from normal trades
mind you, you cannot stop some realm subsidising food trade to another realm (1 way traffic) which may or may not then sell it off for huge profit to some 3rd party and subsidising food is conceptionally no different from giving gold to another realm