MMH5.5 - Heroes of Might and Magic 5.5

at the very least, the developers provided tools for mods or something like that.

Now, comparing mods of a game where the developers intended to provide a platform for Creation, and mods of a game that the developers simply made as a game (bought, completed the storyline, beat the AI on maps, deleted) where modding was not even considered.

You are wrong here.)
The developers of Warlords provided almost no tools, except for the in-game scene editor (which, in terms of importance/functionality, is even less than the map editors in HOMM). All other tools - OpenBrf, global map editors - were created by players.

Plus, it is physically impossible to force someone to ban something. So, there is nothing to stop it with, you can only persuade them with a better alternative, but this does not really work.

No, but why? Take, for example, ModDB, where some mods are uploaded. As far as I know, their Technical Support responds to complaints about resource theft (at least, I know that mods that tried to steal resources from RoR were removed from ModDB). But, of course, it must initially be your resource from start to finish. "I took it from the game, and he took it from me" - well, that won't work.
I am mostly a scripter, so my texture and model work is mostly rework of various OSP. And there were a couple of cases when they were stolen, but I don't even want to complain about it, because even though I spent a few hours modifying some model, its original author spent much more time on it, so the rights mostly belong to him, and he posted it as an OSP.
But in any case, scandals and accusations of theft (up to spamming with such accusations) are not needed by anyone, so with due persistence and complaints, I do not see any particular problems in getting your resources removed from someone else's mod (the scandal surrounding the Chinese Splendor - a submod for RoR - led to the fact that even on the Chinese forum, the mod was banned for some time and eventually its authors gave in and at least partially removed the resources taken from other mods).

Regarding visual mods for HOMM5 and the formation of a sound policy regarding the use of resources, I can't say or recommend anything, because personally for me, they are superfluous in principle. I like the creatures from HOMM5, I don't like 6 and 7, and I'm not particularly impressed with all sorts of repaints (this is perhaps the most "legal" way to start your modding "career"), when some archmages are simply repainted in black and called some necromancers, or the knights of Isabel are repainted in green. In my opinion, most of these mods do not fit very well with the atmosphere of the 5th Heroes.
Personally, for me, it would be a priority to fix the stupid, as a log, combat AI in "Lords of the Realm". You give the opponent a bunch of mana and "magic", spellcasting all-seeing, and all 3 mass perks of light magic, and what does he choose from all this? That's right, he starts casting "evasion" one by one.
But this is difficult to fix without the source code of the engine. (
In general, I tried to redo the campaign (I wanted to make the transfer of set artifacts in all parts + merge 3 campaigns together so that various Railags, Isabels and Fridas are really ported from one to another + add mentors to the maps + many small fixes), but in the end I gave up on the last mission of Findan, because the desires diverged from the possibilities: too few available operations and from a scripting point of view, it is not possible to interestingly implement what was intended.

MMH5.5, by the way, I like in the sense that it adds new artifacts and brings some variety, but I don't like that it is too focused on Multiplayer, and therefore nerfs everything that can be nerfed, including my favorite center perks. :(
And the crooked implementation of some perks leads to their easy abuse in campaigns (another evidence that the developers are only interested in multiplayer), when the bonus stats for the perk are repeatedly given to you at the beginning of each mission, and instead of +2/+2 to attack and defense, you eventually get +10/+10.