Can you stifle stasis?

[Nevermore]

Stasis:
1u 1colorless enchantment
oracle text:
Oracle text: Players skip their untap steps.
At the beginning of your upkeep, sacrifice Stasis unless you pay U.

Stifle: instant
1u
Oracle text: Counter target activated or triggered ability. (Mana abilities can't be targeted.)

I was playing a game of magic fooling around with a stasis deck and i was playing against fish.  I had him completely locked out except for one mana, and on his upkeep he says hes stifling the stasis so he can untap.  I didnt think he could do that because there is no trigger, players simply dont untap.  Is that legal?

Another question that I have is can the "sacrifice stasis unless you play u"  be stifled?  Not that it is a useful play, im just curious.

Thanks

[T00L]

You can stifle the payment cause it triggers you can't stifle the removal of untap steps because that is static

[parallax]

"Players skip their untap steps." is neither an activated ability nor a triggered ability. It doesn't contain a colon or the magical words "at", "when", or "whenever".

"At the beginning of your upkeep, sacrifice Stasis unless you pay  ." is a triggered ability. It starts with the word "at". However, Stifling that trigger is on the same level of play as Spell Snaring a Daze.