What is the proper way to notate the end of a section played "sul ponticello"?

Question

That is, what is the standard way to tell a string player to play normally again?

Answer 1

You can use ordinario or normale:

ordinario or normale: to bow in the ordinary or normal fashion, canceling a previous instruction to play s.p. or s.t. abbreviations: ord. ; norm.; N. - source

Answer 2

'loco' is also used in these situations.