Excel tables on the web are constantly styled. There is no method to delete every style. The most fundamental style, "none," doesn't seem to be assignable in any way. I'll check in on that.
The closest I can get to was to use TableStyleLight1 style as shown below.
function main(workbook: ExcelScript.Workbook) {
let selectedSheet = workbook.getActiveWorksheet();
let mainTable = selectedSheet.getTable("MainTable");
mainTable.setPredefinedTableStyle('TableStyleLight1')
}