Arresting a foreign noble is not an act of war. I can arrest an infiltrator who tries to steal gold without declaring war.
Furthermore, as I've said, the current system doesn't create more wars. It creates less wars, because it raises the cost of provoking wars. Yes, it raises the benefits of war, but that is far less important than the question of the costs to make the actual transition. And, as noted, it means that only a ruler can do anything about a war (click "declare war") while, if arresting were possible, anybody could provoke a war.
You can catch an infiltrator in the midst of committing a crime. Such as changing signs, stealing gold from the tax office or assaulting a noble for example. You cannot order your unit to arrest an infiltrator who happens to be passing through your lands and not committing any crimes. Further, lets say that there are only two nobles in a region. Realm A and Realm B. The noble from realm B assaults the noble from realm A and does not get caught, but successfully assaults the noble. Now, you can infer that the noble from realm B is an infiltrator and you can infer that they are the one that is guilty of the assault. However you cannot send a police unit to the region and push a button to arrest the Infiltrator.
If a priest tries to do something that provokes the commoners of a region such as, Auto Da Fe or declaring yourself Lord of a region and a few other commands like that, then the priest has a chance of being arrested just like an Infiltrator who assaults, steals, etc...
A priest preaching is the lowliest action that a priest can take. They could be making the peasants hate you or lower their morale or building a shrine. Preaching isn't something that you should worry about. It is what happens later, after the preaching that you need to worry about.
