So, where it is explicitly written down, I have no idea. It is probably somewhere. I imagine the GM handbook is much larger and nuanced than what is on the website. For me, however, it's just one of those rules that seems to pass down through the years. And, yes, sheafs, chests, wooden boxes, all count. It's always been understood that if you're impeding the movement of a mob/player, you're going to get a talking to.
I did a quick look at the FAQ on the website:
Rules of Conduct – Ultima Online but didn't see anything specifically related to this. That, again, does not mean it's not somewhere.
I remembered that Mesanna herself (in one of her 9 posts on the main forum) did once comment that intentionally item blocking players on Siege Perilous wasn't allowed.
would you consider this legal or illegal Take that for what you will. Whether or not you agree with her statement (and a lot of Siege players didn't) I think you could infer that if it's not allowed against players in Siege of all places it's not allowed for PVM in prodo shards.
It sucks that you were the one who got noticed when likely a ton of others on your shard and others were bagballing that very moment. If it's your first rules offense you'll probably be back in the game in hours, if not a day.