There's a hint right there if you scroll down a bit:
Adding / removing rows directly in the grid for infinite scrolling is
not recommended as it will complicate your application. It will make
your life easier if you update the data on the server and refresh the
block cache.
And then scroll down to the "Insert And Remove Example", there's example code showing how to remove rows (with a mocked up data source):
function removeItem(start, limit) {
// here you would make a call to your data source to remove the rows
allOfTheData.splice(start, limit);
// get fresh data from the source
gridOptions.api.refreshInfiniteCache();
}
Try to read the documentation more carefully, you've asked a couple of questions that are already directly answered in it. It's all literally on the page you linked to.