There where a small number of posts I didn't read, but here is my suggestion:
For MP3:
I disagree with removing the ability to access the MD Shop, as that will not help them at all. What I suggest is limiting their access so they can't get the Rustgold or TS, or the Ritual of Self Oblivion, or the more powerful tokens. This can be achieved by adding an MP limit to those items, and an MP limit to receiving those creatures via CTC.
I also suggest removing access to the experience reset Wish, or limiting it to one. Again, can be done with MP level requirements.
I think Fyrd's suggestion of removing sacc rewards is good, but I think temporary effects should still apply. I also think that the experience loss from sacrifice should be removed for MP3. I think there should be a notice on the Fenths saying that when advanced you can get permanent stats from sacrificing.
This doesn't limit game play much, and allows MP3 to be able to see the benefits of advancing (more shop access, wishes, and stat gaining methods)
For MP4:
I would suggest just limiting the amount of stats gained per sacrifice (for the stats that have a variable increase based on how you raise the creature), and leaving everything else the same. There are a lot of players who play MP4 as an actual part of their role, and they shouldn't be restricted.
This allows players to get a further understanding of the tools they have, a wider access to creatures, and even for those who stay MP4 for their roles (Z, for example) to not be restricted (since few of the MP4 who are there for roles seem to have much interest in combat)