Earlier quoted context omitted.
You can put "Users may not breathe while using this website" in your ToS but that doesn't make it enforceable. A ToS has to have some basis in law.
Ok... again, not trying to be combative but: a) This still doesn't really answer my question. b) Would it be illegal to put up a website and say that essentially no one can use it? Aside from some exceptions like discrimination, ToS's define how you can use a tool, which is exactly what they were doing. Is there some law stating that if you host a website, you must allow users to link to it in any way they like?
TOS enforcement stems from contract law. A principle of contract law says that a contract is binding on both parties if they agreed to it, but there must be SOME benefit to each party. A TOS between a website and its users clearly meets this threshold, but in this case agiletortoise didn't gain any benefit from the website -- he simply wrote a program that directed the user's browser to go there.
That being said, the best solution is the one that agiletortoise used: simply direct people to someplace less hostile, like Google, Bing, or Wikipedia, instead of Wolfram Alpha. Wolfram Alpha loses, everyone else wins.