is stalling a ban worthy offense?

Out of curiosity, recently I have seen opponents stall the until their time runs out simply to not get checkmated. Just resign? No reason to waste both player's time 

You should report them. There's a specific tick-box on the report form for "Stalling".

Not considered stalling for daily games. Consider it stalling for rapid, blitz, and bullet games.

I don't think it should be ban-worthy, but if they do it too much and get reported, I think they should have repercussions.

Not sure how to punish normal games, since the only thing I can think of is to force abandon if they waste, say, 30% of a game's time, but that could be considered highly unfair in tricky positions that need a lot of thought.

However, I think stalling in Arenas should be met with banning from joining Arenas for a specific time. To me, it is a much more serious offense, since you are stealing time from the opponent's next game and thus stealing time for them to earn points during the Arena.