So my question can we activate Trap Hole against Denko Sekka?
The reason is because Trap Hole effect activates after the summoning of Denko Sekka, thus her effect is already in place. This is exactly the reason why Denko Sekka's effect is powerful.
Reading Trap Hole's effect, emphasis mine:
When [that is after the summon is successful] your opponent Normal or Flip Summons 1 monster with 1000 or more ATK: Target that monster; destroy that target.
You could, however, negate and prevent her summoning altogether, with cards like Solemn Warning as you correctly pointed out. From its text, emphasis mine:
When a monster(s) would be Summoned, OR when a Spell/Trap Card, or monster effect, is activated that includes an effect that Special Summons a monster(s): Pay 2000 LP; negate the Summon or activation, and if you do, destroy that card.