To delete the checkboxes you have created, hold down [Ctrl], click all checkboxes to be deleted and hit [Del] or [backspace].
You may find it easier to delete via the "Selection Pane" in "Editing" - "Find & Select". Select and delete the named check boxes in the same way as above....
For hundreds of checkboxes you can also try...
Use the "Selection Pane" above to confirm that the only objects in your sheet are checkbox objects. If they are, then use "Editing" - "Find & Select" - "Go To Special". In the pop up box, choose "Objects" and click OK to select all checkboxes (then hit [Del] to delete them)...
If you need to be more selective you can drag the mouse to select the range containing checkboxes using "Editing" - "Find & Select" - "Select Objects"
If all else fails, resort to VBA... "Developer" - "Visual Basic", "Insert" - "Module"...
Sub RemoveCheckboxes()
On Error Resume Next
ActiveSheet.CheckBoxes.Delete
Selection.FormatConditions.Delete
End Sub