If you actually had mutually exclusive effects (e.g. "X must block Y" and "X can't block Y"), those limiting what you can do would win over those that permit you do something. This is colloquially known as "can't trumps can", and it's the second Golden Rule of MTG.
101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
More specifically, you need to comply with the most requirements ("must") as possible while complying with all restrictions ("can't").
508.1c [...] If any restrictions are being disobeyed, the declaration of attackers is illegal. [...]
508.1d [...] If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. [...]
However, neither of the scenario you posted actually have mutually exclusive effects. One is quite capable of obeying both restrictions.
"Juggernaut can't be blocked by Walls" and "Equipped creature can't be blocked except by Walls" are not mutually exclusive. It's possible to obey both restrictions by not blocking the Juggernaut.
"Charging Rhino can't be blocked by more than one creature" and "Enchanted creature gets +3/+3 and can't be blocked except by two or more creatures" are not mutually exclusive. It's possible to obey both restrictions by not blocking the Rhino.
One must obey all blocking restrictions[CR 508.1c], so one would not be able to block the Juggernaut or the Rhino in the scenarios you presented.