How can I prevent monsters from being attacked in scripts?

You can assign a trigger to a monster, and disable the monster itself as an object.

From the manual in my signature:

How to perform an action when touching a specific monster?

As a rule, a monster with non-standard behavior should be visually different from a
normal one, so as a bonus to the touch trigger, I add changing the cursor from the
standard sword to a peaceful horse, removing the monster's "skirt", and changing its
name:

-- setting up creatures-NPCs on the map
-- npc – the name of the monster assigned in the editor (string)
-- func – the touch handler function (string)
-- name – the name of the file with the creature's name (string)
-- func and name – optional parameters, they can be omitted
function SetNpcFunc(npc, func, name)
  -- disabling the standard monster touch trigger (combat)
  SetObjectEnabled(npc, nil);
  -- setting a peaceful cursor when hovering over the monster
  SetDisabledObjectMode(npc, DISABLED_INTERACT);
  sleep(1)
  -- disabling the "skirt" (faction symbol under the monster)
  SetMonsterSelectionType(npc, 0);
  -- setting your own touch trigger (optional parameter)
  if (func ~= nil) then
    Trigger(OBJECT_TOUCH_TRIGGER, npc, func);
  end
  -- setting the monster's name (optional parameter)
  if (name ~= nil) then
    local full_name = GetMapDataPath()..name;
    SetMonsterNames(npc, MONSTER_NAME_SINGLE, full_name);
  end
end

-- examples of using the function:
-- the monster simply stops reacting to touches
SetNpcFunc('SCRIPTED_MONSTER_NAME');
-- a touch trigger is assigned to the monster, the name remains the same
SetNpcFunc('SCRIPTED_MONSTER_NAME', 'NpcTouch');
-- a touch trigger is assigned to the monster and it is given a new name
SetNpcFunc('SCRIPTED_MONSTER_NAME', 'NpcTouch', 
'FILE_NAME_WITH_NAME');

-- monster touch handler
-- hero – the scripted name of the hero who touched the NPC monster
-- npc – the scripted name of the monster that was touched
function NpcTouch(hero, npc)
  YOUR CODE
end