Akismet cannot be turned off on wordpress.com blogs. By design, Akismet will delete any comments it catches or believes to be spam after 15 days, unless you go in and "despam" them before that point. Once they are gone, they are gone.
In general, you should check your akismet spam filter in your dashboard > comments > akismet spam every day or two so that legitimate comments can be identified and despammed.
Here are some additional general points worth knowing:
1) If multiple identical spam comments are sent to your blog, the duplicates will not show up in my spam list for you to review.
2) After you have verified that the above spam is indeed spam by deleting it and Akismet has learned, if you again get hit by the same source, your "spams caught" number will increase, but they will not be listed for you to review (since you have already identified them as spam).
3) Any comment sent to you blog, that is already known by Akismet to be spam, will be blocked, your "spams caught" number will increase, but again, they will not be listed for you to review (since they have been previously identified as spam by yourself or others).
4) If a legitimate comment is caught in your spam filter and you "despam" it, it may take up to a few days for your despaming to work its way through Akismet and if you get a comment from the same source in the meantime, it may still end up in the spam folder. If this happens, despam it again. Akismet will is a short period stop catching them.