Ignore feature tutorial: find the name of the mult
Post# of 72440
Click on that name (it should be highlighted in bold or a color, indicating that it's a link). That will take you to the identity's personal page.
On their page, look for the avatar. (on my computer it's on the left side of the page, I don't know where it displays on a mobile device.)
Look underneath the avatar. About the 3rd link down is the "ignore poster" link.
Click that, and problem solved.
You can also "block poster." This prevents a multi-identity from sending you Personal Messages.