I'm not sure if "mods" are the ones managing communities, while instances are managed by "admins" or similar.

On reddit, sub mods can ban you from their sub, and reddit itself is the single instance with its own guidelines which can get you banned from the whole platform, with the possibility that in the future we'll need identification to login, so creating multiple accounts might even become something of the past

Lemmy is a protocol, every instance should be seen as its own small reddit, which can behave like reddit, however all these reddits are connected together so even if one provider manages to ban you and your multiple accounts on that instance, it has no legal responsibility on your activity on other instances. There is no central Lemmy authority that could be legally compelled by the laws of one country to have specific moderation policies.

If somebody's behaviour is terrible, like idk fascist propaganda, they will be banned by every instance, except those with inactive moderation or fascist admin/moderators. In this case, other instances can (topically collectively) decide to sever the federation to those instances. Thus, a general ban on individuals is possible, but only by a collective effort of people who have otherwise no legal obligation to each others