Here are a few options:
- See the BBEG through divination such as scrying so the players can't interfere. Also works if they're in a far off location and can see the chaos the creature sows.
I've used this to give characters with spying and social networking skills a way to get useful and plot focused information.
- Introduce the BBEG in a setting where etiquette, personal codes, etc. are the problem.
Notable mentions are court sessions in L5R or Sidhe realms of 7th Sea. A territory can have binding rules that are as soft or hard as you like and prevent characters from diving in. This can be as mundane as a formal ball and not wanting to lose royal favor, as metaphysical as Geas or losing a protective magic like being invisible, or as literal as a fight causing a cave in.
- If the creature is intelligent, allow them to bargain the players away for now.
Bribery and being told of a bigger threat (for now) could deter the players. In a recent game, the players had to decide if the lord hosting their stay was lying about a curse on the land, and decided to take a simple task from him for his aid until they could learn more.
- If it's a "Good" party, they shouldn't be murder hobos and won't just attack something for existing.
The older I get, the more even my most warrior based tables try to avoid a fight. Naturally, once it's on they don't hold back, but it's useful that they want an actual reason to kill something. I've also posed a moral dilemma to a paladin about whether or not they should kill a hatchling black dragon. It's an evil creature, but it also hasn't committed any crimes yet.
- Put the initial encounter when the party is ragged from the rest of the dungeon/encounter/scene. No special abilities and low health should make them unlikely to attack something they don't know
This is one I've done in war campaigns. The players are just at the end of a battle and they could take this foe on now, but they are simply out of gas and need to pull back if they don't want to whip out the big eraser. It created for a fun tug of war mechanic because they would take an area but have to return to a base camp to recover, which not only gave them glimpses to the next threat, but me time to plan it better.
- Competing interests are always valid. If the players have to bring the MacGuffin back unharmed, and the create is a cut scene in their path they'll keep walking.
Part of the socially binding limits, collateral damage is always a factor. Sometimes the creature needs to be let go and continue its rampage because it takes it away from the town. I've also had a campaign where players were in house that was collapsing from being dilapidated. They were in there to recover a family heirloom, which was a painting, and thus easily damaged if they got into a fight. They also didn't want to risk a TPK by staying somewhere unstable.