Some people recommend going through QuickBooks annually and marking vendors you no longer use as Inactive. The benefit of doing so is essentially to clean up your QuickBooks file. Also, inactive vendors don't show up when you are entering bills, decreasing the chance you will accidentally make a bill out to the wrong vendor. Given that it is incredibly easy to reactivate an inactive vendor, there is definitely no harm in doing this. That said, I don't generally recommend it.
The two reasons I don't generally recommend spending time marking inactive vendors as inactive are:
I say I don't generally recommend spending time marking inactive vendors as inactive because sometimes I do. If you know a vendor is out of business, then take the couple seconds it takes to mark them inactive. If you want to re-enter a vendor with a slightly different name, but don't want to delete or use their old information, then mark them inactive. Therefore, mark vendors inactive on a case by case basis using common sense.
Finally, to mark a vendor inactive, open the Vendor Center and right click on the vendor you want to edit and select Make Vendor Inactive.