Isolating that U.S. of A. culturally, politically, and commercially from the rest of the world would have a barely noticeable effect on the speed of evolution of the American language.
All languages evolve. Linguistic evolution cannot be stopped. It can be slowed down. Mass literacy and pervasise nation-wide news and communication media are factors which tend to slow down the speed of language evolution.
American English does not have regional dialects. There are regional differences in pronounciation, but that's benign and expected in a country as large as the U.S.A. On the other hand, American English does have cultural dialects, also known as sociolects. For example, Black Vernacular English is a strongly marked dialect, which some will say it's well on its way towards becoming a separate language altogether; it already has specific grammatical features which distinguish it from standard American English.
American English is already visibly different from other varieties of English, such as, for example, Indian English or Australian English. One century of isolation will only increase the differences. At what point it will become generally agreed that American and European International English are two different languages is partially a political and cultural question.
For example, in South Africa, Dutch became Afrikaans, in about two centuries. Everybody agrees that Dutch and Afrikaans are two very closely related yet different languages.
To make American split into two or more daughter languages you need to split the U.S.A. into two or more countries which don't communicate much; a sort of a cold war would help. If you cannot or will not split the U.S.A., then lowering the rate of literacy and breaking up national news networks will help too. Isolation does not have to be absolute; all that's need it to make sure that the average person from, for example, Seattle doesn't ever communicate with a person from, for example, Dallas. You do need to keep up this isolation for ten generations or so.
Each successive generation speaks a language which is slightly different from the language of their parents. The difference is small, but it's real and measurable. If two linguistic communities are relatively isolated from each other, that is, members of one community don't have to communicate much with members of the other, then in time those differences will accumulate and result in two closely related yet different languages. If the isolation is maintained then the distance between the two daughter languages will grow to the point where communication becomes impossible without translation.
If you can imagine a way to divide the U.S.A. into two or three relatively isolated linguistic communities you may well end up with a situation where each such linguistic community speaks its own language, with educated people speaking standard American English in addition to their local language.