I'd start to have my monsters NPCs find other ways into the encampment; using trees (Climbing up then shimmying across branches to drop down into the camp), flight, rings of blinking, etc.
I hate Borg tactics.
Somehow these enemies who never would have dropped into camp via tree branches suddenly start doing it. Why? Apparently they just "know" that there is a hidden moat. The knowledge is just shared among all enemy creatures, like a hive mind.
I prefer that my monsters are not mind-linked. They do not know that the previous group was spiked to death while trying to engage the party. They may be spiked to death too. If so, tough luck for the monsters, and kudos to the party for finding a winning strategy.
If this means that my wandering monster attacks dwindle down to a minuscule part of the game, so be it. The players will be engaged in other ways.
I dislike similar handling of successful feat combos. The player did the spiked-chain-combat-reflexes thing and all your monsters are getting walloped? As the DM, your job is not to "put an end to that" any more than you should be putting an end to the fireball spell.
Some things are successful. That's why they get used. Nerfing these things undermines a player's choices.
That's not to say that you blindly put every monster into the stew pot. The players will eventually get an encounter with stirges or something that bypasses their defenses every now & then. The trick is just to do that only at the same rate that it would have happened normally. That may mean lots of hobgoblins falling into spiked pits over and over again.
At one point my cleric used Glyph of Warding spells with sonic damage -- the natural assumption being that if it is so loud as to damage someone physically, it ought to be loud enough for the party to hear. We would make camp, spread multiple glyphs out around the perimiter, and all sleep without watches.
Time and time again, our DM had us being awakened by loud blasts. The monsters rarely died, but always entered into combat with us in wounded shape. Our battles were easier because of the work we did on our spell-ish imitation of a moat.
If my DM had not allowed that to play out similarly most times, I would have accused him of metagaming.