I think it would be better if you expose few of them who PM you for a merit. If you publicly announce who are PM you for merits, they won't get the merit from anyone and it will be a good lesson for them.
Also it will prevent people from further asking for merits in the inbox and this trend will soon end. There could be a thread where everyone could report who is privately asking for merits.
There might be some who might oppose, claiming privacy of PMs should be respected #PLM.
Ironically it happened in the Wall of Shame thread not too long ago.
You've to agree that not all the messages contains with value, purposeful, encouraging some are pathetic as well. For instance, If you have received such begging PMs then you just simply put the member on your ignore list just like OP did. Sound agreeable, nah?
Some member need to grow up, as merit things make them impatience. However, Newcomers should keep the privilege of curating the discussion, for everyone's benefit.