Usually, what's being argued about in the arena of beliefs is neither provable nor disprovable.A person's beliefs might fly in the face of fact, or said person might need to misrepresent or distort the claims of others in order to make their beliefs seem correct.
Unless you've shown said person time and time again examples of events that actively contradict their beliefs (or claims related to such) or show them to be false and they continue to expound as if you'd never shown them anything.
Calling a duck a 'duck' isn't an example of bad behaviour. Sometimes 'liar' is as good a word as any to describe someone's dishonesty.
You think socialism sucks?
Or you think capitalism sucks?
These are value based opinions.
I've never seen anyone prove someone else a liar.
Suppose I tell someone over & over that they're wrong about something....
If they don't change their mind, this isn't necessarily lying.
What's so obviously false to one person isn't always so to another.
I've been thru this before.
Someone tells me how dishonest I am, based upon their claim of "facts" & "logic".
The reality is that they offer neither....just opinion mistaken as fact, & reasoning which really just presumption.