Dealing With Character Strengths

Suppose the scout in the adventuring party is specialized in awareness and ambushes such that only a few monsters or situations can ever surprise the party.

The worst thing you can do:

Take the scout's optimization as a personal challenge and optimize all encounters to negate the party's advantage, or even reverse it!  Since you are the DM, you are all powerful, and beating a player character at something is not an accomplishment.  Very little breeds bad blood in the group more than a competition between the DM and the players.  And, since the player worked hard to build the scout, the negation of all the player hoped to achieve will result in that player requesting a new character or, worse, leaving the group and hate you for the rest of your life.

Another way to get it wrong:

Be lazy, or oblivious, and never consider stealth and perception rules and just have all encounters start with both parties aware, maybe even start them at fixed positions you decided when you.  This attitude still negates the scout's labor, but won't make the player nearly as angry.  The player will be bummed, and might complain, retrain, switch characters, or all of the above---but probably won't leave you.

A non-ideal, but not-quite-as-bad case:

Give full consideration to the stealth and surprise rules, but don't respond to the scout's optimization.  The party will have surprise almost every encounter.  If it continues, they will front-load their powers and make your encounters very, very short.  If you respond at all, it will be by raising the difficulty of the monsters, which is going to eventually start risking total party knockouts, which are a pain.

A good case:

Fully cognizant of the scout's optimization, you intentionally let the party have surprise more often than a normal party (which is easy because it will be the natural course).  Additionally, you sometimes design encounters that obviously depend on getting surprise. That way, they player's feel rewarded for their design work on their characters.  On the other hand, you sparingly use terrain, weather, deception or ritualized combat to complicate advanced scouting.

Conclusion:

The bottom line is, as a DM, you are a fun enabler.  You should be spending your time making the players' hard work pay off, not trying to compete with them or work against them.  But, sometimes you have to protect the party from itself by kicking it out of a rut.