What should you do if the surplus worker level limit is exceeded?
Following the suggestion given by user @Oltarus, I contacted Ludically and got the following answer from Christophe Boelinger, the designer:


  Do nothing.
  
  When The surplus worker reaches the max level then it stays there no matter if any effect would require to go higher, and in such a case nothing else happens than leaving the marker at the max level. Which is already not a very helping situation anyway.
  And this no matter if it is in a solo game or multi player game.