I'm posting this issue as a result of #1293186-61: Spam - meta: better spam-combating suggestions.

There are some tables in notifications that don't have sufficient indexes which results in user operations like blocking not actually working.

I suggest deploying the indexes mentioned in #868446-7: Improved indexes for Notifications during: Notifications cancelling/User blocking/User delete operations. Its possible those indexes have already been deployed and the root cause is something else.