First print and post all the invoices that have real shipper numbers and dates that you want to keep.
 
Then UT-K-B, file BKARINV.
 
First for field BKAR.INV.SHIPDT, enter value to replace and enter today's date (assuming this happened today and that is the date you want to get rid of), and for the value to replace with enter 00/00/00
 
Then same file, field BKAR.INV.SHIPPR, enter value to replace as blank and say Y to replace all values, value to replace with is also blank.  This will clear ALL the shipper numbers which is why I said you need to post the good ones first so the shipper numbers are saved in the Invoice file.  If you want to clear only those shipper numbers greater than some number, then IS Tech has a search and replace utility for $49 that can do that but if you can post all the good invoices first, you really don't need it.  Finally, go back to Sales Order sefaults and reset the Next Packing Slip number back to what it should be.