In a case where you're NOT seeking the death penalty, can you prohibit Defendant from going into parole law during voir dire ?

Assuming you are referring to a case where 12.31(a) applies, then there will be no punishment phase and the jury should be instructed both in accordance with subsection (b) and that "in determining the guilt or innocence of the defendant, you shall not discuss or consider the punishment, if any, which may be assessed against the defendant in the event he is found guilty". The only purpose of mention or questions about 508.154(b) of the Gov't Code would appear to be to encourage the jury to return a nullification verdict as to guilt because of bias against 508.145(b). Seems to me you could either seek to qualify all jurors that the 40 year minimum will not affect their verdict (i.e, that they can follow the above instruction), or you can take the position no mention of 508.145(b) should occur from either side. But, since there may be an instruction under 19.03(c), some mention of the parole law applicable to the lesser offenses may be required or proper during voir dire. Sells, 121 S.W.3d at 756 looks like a good place to start research.