I want to make a container enterable, but I don’t want the player to be able to type “enter [container]”. I want them to have to type “climb [container]” or “board [container]”. What do?
That sounds like a bad idea. ENTER is such a standard verb that if it’s not allowed many players would just assume that the thing can’t be entered or that there’s a bug in the game. What would you gain from disallowing that command?
Ideally I wanted to prevent people from just going “enter [object]” because the object is a solid mass that cannot be “entered”, it can only be climbed upon. I wanted to make it a very insignificant world puzzle, essentially. I’ve tried using one of the Al Golden extensions but that caused too many problems.
Are you sure you don’t want a supporter rather than a container? It sounds like the player will wind up on top of the solid thing rather than in it. Even there, Juhana has a point; lots of people get onto chairs and the like by typing “enter chair,” so disallowing enter might confuse them. I do sometimes try climbing things like trees when “enter” doesn’t work, but if the puzzle is supposed to be insignificant there’s not much point in confusing the player here.
Another point: “get on cheese” maps to “enter cheese,” I think. If you just disallow entering, you’ll block “get on cheese,” and that will really confuse your players. (Also, unless you’ve set things up very well, I don’t think anyone is going to try “board cheese”; that seems very guess-the-verby.)
If you really want to disallow “enter,” maybe you could try something like this:
Instead of entering the cheese when the player's command includes "enter": say "The cheese is solid. You might be able to climb it, but you can't enter it."
Instead of climbing the cheese: try entering the cheese.
Understand the command "board" as "climb."
Totally untested, and I’m not sure the syntax works; also I can’t remember whether “climb” is a built-in verb/action.
If the thing is clearly something that can’t be entered, why would players then even try to enter it? 
I would still allow ENTER but with a clarification of what the command was translated to:
Before entering the cheese when the player's command includes "enter":
say "(by climbing the cheese)[command clarification break]".
I’m with Matt that – if this is the cheese thing from the other thread – you want an enterable supporter rather than a container. The upshot is that with a supporter you don’t have to worry about text like “You can see a cheese (empty) here.”
My normal policy is to refrain from giving design advice and just answer the technical part of the question, but in this case I have to agree with the other guys; player want more synonyms, not fewer. A few years back a game had a torch with a bracket that was implemented as a supporter rather than a container, so you had to “put torch on bracket.” “Put torch in bracket” resulted in the standard “That can’t contain things” message – a fact that more than a few reviewers pointed out with almost gleeful disdain.
As far as your actual question, the easiest way is probably:
Understand "enter [cheese]" as a mistake ("You can't enter the cheese, but it looks like you can 'climb' or 'board' it.").Once again, please don’t do this. If your players are like me, an example of a likely transcript would be:
Hmmm, I guess you are all correct that it would be a poor design decision. Guess I’ll give it a miss this time around, but thanks for the detailed answers anyway!