Let me start with a little “secret”. In people’s make pretend, they usually want to succeed. If they imagine a smooth talker, they want to be that. If they imagine a master swordsman, they want to be that. However, in D&D games, people having that level of competence in the way that it’s expressed, usually in big numbers, sometimes in absolutely impossible tricks is terrifying for a lot of DMs. Some will try to avoid that player using the big numbers. Some will outright panic when they hear the number. A few will start screaming down the player and force them to play another character. None of these really handle what you want to do well, though.
Before I really start, I do want to say that even following the advice here, you still have the right to say that a character can just do too much. If you can’t keep up with it, or if it’s marginalizing other players, talk to the player, see if you can tone things down where the character can be really good at what they want, but not overwhelming. Maybe that +30 Bluff item becomes a item that gives Tongues and +10 Bluff, letting them both do their job better and not as well. At the end of the day, remember that a game needs both Players and a DM to keep going.
With that disclaimer out of the way, your first goal is to figure out why you’re staring down the huge number. The player that’s running the huge number can do it for 2 major conflicting reasons. First, the player simply wants to trivialize that part of the game. It’s cool if he can show off the skill once in a while, but the player with +50 to Swim may just be saying “No more swim checks.” On the other hand, you have the player that wants to be able to accomplish the impossible. Maybe the level of convincing a high ranking Devil to become truly good. Maybe it’s being able to walk on clouds. Maybe it’s something else entirely. The first task is to figure out why they have the high skill check. While I would advise just talking to the player, if that’s not possible, you may just have to prod a bit.
Now the guy in the first group is easy to appease. When you call for the check, unless it’s really hard, just tell him he succeeds. Ironically, if you have this kind of player, you want to plan an out if you need a check in his speciality to be hard for him. Even if you don’t plan one out, expect the player to move the skill to “easy” before engaging with it. Generally, the difficulty in the task should be getting the less skilled people through, not what this player would have to pull off.
The second one is much more interesting. Pull out a lot of “You succeed on the easy stuff”, and he’s going to become disengaged. He didn’t power up to trivialize the small jumps and bluff the peasants, he did it to go soaring and to get the kings to hand over ransoms. In a less sandbox playstyle, if you put the task in this guy’s speciality out there, he will try it, maybe directly, even if it’s not an appropriate task. Don’t be afraid to let this guy sometimes be the quick way out of something that should be nearly impossible except through a long quest. At the same time, there have to be events which can make him fail, even if very unreliably. The challenge is the important thing.
Now, the right answer to bigger numbers is never to jack up DCs without an actual reason. A curse over a town that compels people to blurt out the truth? Good reason to up the DC. Commoners in a tavern talking quietly? You don’t jack that up for the 10th level party coming in, and you don’t jack that up for the guy with +tons to Perception. This is the quick way to make your world feel hollow and to make progression seem uninteresting.
One final thing to remember, if a player’s good at something, it could start acting as a hammer to every nail. If you start seeing this, you want to force them to put more discretion to it, and make it a viable indirect answer to some of the problems. The guy with a lot of Knowledge (Planes) should eventually be able to bypass the riddle that your players are having no fun with. Convincing a Fire Elemental to follow you in the dungeon might be the way past the murderous Fireball trap. Maybe you meant them to climb out of the pit, but your miner starts tunneling into a side room. With really capable characters, you rarely have to worry about your traps and puzzles having no solutions, even if you can’t dream one up.
In the comments, I’d like to hear about reader’s interesting ways of utilizing characters with some sort of check at +yes, or a skill/capability that they feel is still going to be really hard to deal with at +yes.