In the case you want to format it within a formula (for example, if you are concatenating strings and values), the aforementioned format option of Google is not available, but you can use the TEXT formula:
=TEXT(B1-C1,"HH:MM:SS")
Therefore, for the questioned example, with concatenation:
="The number of " & TEXT(B1,"HH") & " hour slots in " & TEXT(C1,"HH") _
& " is " & TEXT(C1/B1,"HH")
Cheers