Can someone please clarify.

When asked to spawn a monster (ie zombie) which is happening nearly every turn in the map currently playing what do you do when group limits are already on the board.

Reading POD rules the monster is pulled off the board, recovered and placed at the specific placement as per the new instruction.

However cannot find in original rules or the Road to Legend rules.

Do you disregard and not spawn any more or do you remove from the board and replace?

That's a really good question, we just have encountered the same problem yesterday.

A major peril (if I remember correctly was "Shadow Binders") have showed up spawing some monster which in the same turn were required by a tile we had just explored in the quest "Tombs of Kayladorn") and we chose to remove them and replace them in the new room.

However a bug appeared also in the system since, because the new moster group were already in the monster list, the characteristics of the unique monster in the new monster group did appeared. We had to resolve the problem quitting the campaign and killing the first group monster before re-exploring the new room.

As far the rule book goes, you never go above group limits unless stated. No matter what. And you always spawn Master > Minion. I would rule it to not spawn any more zombies if you were at group limit.