I forced some narcissus bulbs to bloom indoors, in water. Such a nice smell
and sight on wintry days. Now that the bulbs are almost "spent" and should be
deadheaded, is deadheading done on forced bulbs, and if so, how? Or should I
toss the bulbs and get new ones for next year?

If you mean 'Paper White' or 'Soleil d'Or' narcissus which are sold for
forcing, those should be discarded. If you mean temperate yellow daffodils,
such as are grown outdoors, you can remove the tops & dry them off a bit. Then
plant them outdoors as soon as possible. If they survive and recover, they will
bloom again in a few years.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)