The description of it as "harassment" is incomprehensible to me. Why post on a discussion board if you aren't willing to handle disagreement, and defend your claims?
TBH, if you want to convince me that someone who is disagreeing with you politely is acting in bad faith, you have an uphill battle.
This can be also a technique used to lure people into giving their private info.
Instead to ask on internet: Are you a republican white? and obtaining a standard "not your business" they claim that you are brown and socialist, expecting you to correct themselves and reply "not, I'm asian and will vote for X" or so.
This way they can obtain the info to classify you without you even taking notice of it. Is an old trick.
What must be taken in mind is that who you are is irrelevant for the discussion most of the time. Facts are either true, or false (or unknown), no matter who will say it.
The "burden of proof" might apply well to formal debates where a consistent set of participants are all acting good faith, but that's hardly the case in most online discussions. Even when people asking for sources are asking in good faith, it is unreasonable to jump into a conversation that you are not educated on and demand explanation from the person you disagree with.