Option 1 did get rid of the autofilter, but it also removed the theme, which I would like to keep.
Option 2 didn't do anything. Immediately after inserting the table, I evaluated the ws.AutoFilterRange property and it was already null.
So, I started poking around a little and found the solution that worked for me:
var table = sheet.Cell(1, 1).InsertTable(dataTable);
table.ShowAutoFilter = false;
Next time, I'll stop being lazy and look around a bit more before posting a question. :)