Deny takes precedent over Grant, so if you only want users of the proc to select, then simply deny INSERT/UPDATE/DELETE on the tables the proc uses to stop them from changing the data.

You could look at the db level role db_denydatawriter which will prevent all Insert Update Delete statements.

But as Lowell said, drop and re-create will remove the existing permissions on the old proc and allow you to put in new permissions on the new proc.