You do have the option to change to a non-unlimited plan to avoid the throttling, otherwise you're stuck with the speeds that you have due to how high your usage has been in comparison to other users with unlimited data.
With the new mobile share value plans available most of the time it's more cost effective to actually change to a plan with a limited amount of data that fits your actual usage, and that way you get highspeed internet no matter how much you're using.