First, B theory is a semantic theory about the proper way to refer to events in time, not a metaphysical theory about past and future events. The view that past and future events are real is called eternalism. It's true that B theory fits better with eternalism than with other metaphysical theories of time, but strictly speaking they are distinct.
Second, eternalism and determinism are also distinct views.
Determinism is the view that the future is determined by the past and by the laws of nature, but it is conceivable that future facts are real although they are not determined by laws (for example: the exact same state A evolves into either B or C at different places or times so determinism is false).
Regarding compatibility with libertarian free will, the crucial aspect is necessity. Libertarian free will is sometimes expressed as follows: an agent could have done otherwise, which, roughly, means that what the agent did was not necessary.
Libertarian free will is incompatible with determinism because if everything happens in virtue of the laws of nature, and if the laws of nature express relations of necessity in the world, then it is not true that an agent could have done otherwise.
However libertarian free will is not strictly incompatible with eternalism.
Saying that future events are real does not mean that they will happen in virtue of necessity so there is room for a combination of libertarian free will and eternalism.
Having said that, this is quite a formal reasoning and our intuitions speak differently. Most defenders of libertarian free will would probably say that it requires that the future is open, i.e. that future events are not (yet) settled for us to have any choice. This intuition can be cast in terms of necessity: one could argue that something that is the case is necessarily the case, so that if every fact is settled in eternity, then everything true (including about future facts) is also necessary, and that therefore eternalism is incompatible with libertarian free will.