Easy, just clear the SPR-RAM by loading #$00 into each one of the bits. I'm not sure what method you are using to load the sprites (DMA transfer is the best for authentic NES coding) but make sure its the right sprite. Graphic #$00 should be completely transparent.
A loop like the one you want would look something like this

loop:
lda #$55
sta $2004
lda #$07
sta $2004
lda #$01
sta $2004
lda horizontal ;horizontal is x variable
sta $2004

jsr DMAtransfer

inc horizontal

jmp loop



Mike Deschenes
http://www.geocities.com/deschenes2020