That's the basic example of the fallacy of moving the goalposts.
JW: not-p because x.
non-JW: not-x; therefore, p.
JW: not-p because y.
non-JW: not-y; therefore, p.
JW: not-p because z.
non-JW: not-z; therefore, p.
No matter what you say, your opponent will think of another (unreasonable) objection. Eventually, it will end with this argument:
JW: not-p because Governing Body said so in one of the publications.
On the other hand, here is what I can say, and this is a syllogism which by definition necessarily leads to the true conclusion:
1) Beards are appropriate for Jehovah's Witnesses in countries where they are accepted by the general population.
2) Beards are accepted in both the USA and Canada by the general population of those countries.
3) Beards are appropriate for Jehovah's Witnesses in both the USA and Canada.
The above is an example of deductive reasoning. You cannot deny the conclusion unless you can challenge one of the premises. Jehovah's Witnesses have to accept both premises (premise #1 is even printed in their magazines) because they are true, but they still refuse to acknowledge that the conclusion is true. This is wrong, irrational and illogical.
So... how do you debate someone who doesn't want to obey the rules of logic? You don't. You let them win and think they're smarter. Having that said, I myself will have to work on listening to my own advice...