Psy.. ffs.. a simple text box in the registration field prolly won't do it. God damn spammers are amazingly clever these days. Try a drop down box as well or another a math problem as in.. 2 + 3 = ..........
maybe that will nip the spamming problem completely in the bud.
Though.. so far the text box you put in seems to have brought the spammers down to a reasonable level at least.