Based on the rulebook context:

• On page 21, it states:
  "You may only defeat one Most Wanted Bounty per Action Phase."
• For monsters, it says:
  "In order to defeat a monster, you must be on the same space as the Monster’s figure."

There is no explicit rule stating you cannot defeat a monster and a Most Wanted Bounty in the same Action Phase. The restriction is only on defeating more than one Most Wanted Bounty per Action Phase, not on defeating a monster and a Most Wanted Bounty.

Summary

A player may defeat both a monster and a Most Wanted Bounty in the same Action Phase, as long as they meet the requirements for each (being on the correct space and having enough Attack), but may only defeat one Most Wanted Bounty per Action Phase.

Source: page 21.

Source: Core Rulebook (pp. 10, 11, 21, 22, 26)