What's the best way to deal with Nasus at the bronze level?

Ban him, average bronze kat player is not a threat at all

I perma ban him as a midlaner lol.

I guess just get a tank shredder like Vayne or Fiora to shut him down if he does get ahead. QSS is good against him to cleanse you from his slow but that's only a "one use per fight" kinda thing.

When his Q is stacked like crazy, he shits on squishies (as you probably know), so picking a comp with at least 2 tanks and a lot of crowd control works well

Kat? Nasus is a 1000x more dangerous than Kat is. Ban him. Kat is a waste of a ban anyway.

picking a teamcomp that's tankier than nasus when nasus is bodyblocked by 4-5 people he can't really do much

If you're not going to ban him (note: I'd ban IVERN over Kat in Bronze tbh. Bronze Kat players exist only to int. >.>) then your best bet if you're playing against him is to take an entire team of people who deal reliable %hp damage in a safe-ish manner. Most of those champs are beefy anyway (the rest are like Vayne, untouchable to him), and pretty much, there's no point in him trying to fight you or your team if you just nuke him through all his resists.