Excel Online... is there a way to stop users from deleting rows/columns but still allow them add and delete text from cells?

Ok, I'm pretty stuck on this one and am not sure why.

I'm using Excel Online (yea, I know... it sucks). I'm attempting to create a filterable table in A1:M1000 that users can enter data into. Where I'm stuck is this... I want users to be able to enter/delete (delete by selecting an entire row and hitting 'delete'), use drop downs, and filter A:M all within that range. I do not want them to be able to edit anything outside of that range (so N -> beyond and 1001 -> beyond) AND I do not want them to be able to add or delete rows/columns across the entire sheet.

My hiccup is coming here... how do I lock the sheet while allowing users to add data to A1:M1000, select a row and hit 'delete', but NOT add or delete rows/columns? I've attempted to lock the entire sheet but unlock A1:M1000; however users then cannot select and entire row and hit delete.

Anyone have any helpful tips for this?