My question is can a HOA board remove the President with a no cause proven?
No cause proven to whom? The answer is to refer to the by-laws, but it is generally the case that the shareholders of a corporation elect the board, and the board then hires (and may fire) the officers.
Also what can be done in this situation as we are being bullied and harassed by those HOA members.
Depends on exactly what's happening. It's possible that you may be able to obtain a restraining order. Consult with a local attorney.
Does anyone know any Pro Bono attorney that can handle this case
First of all, you're not going to get a referral from a message board. Second, there's no such thing as a "pro bono attorney." Some lawyers will occasionally do work on a pro bono basis. However, unless you happen to have a family or friend relationship with the attorney, that typically only happens in the context of civil rights or representation of the indigent.