Turning off some legends in a ggplot

Suppose I have a ggplot with more than one legend.

mov <- subset(movies, length != "")
(p0 <- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
  geom_point()
)

I can turn off the display of all the legends like this:

(p1 <- p0 + theme(legend.position = "none"))

Passing show_guide = FALSE to geom_point (as per this question) turns off the shape legend.

(p2 <- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
  geom_point(show_guide = FALSE)
)

But what if I want to turn off the colour legend instead? There doesn’t seem to be a way of telling show_guide which legend to apply its behaviour to. And there is no show_guide argument for scales or aesthetics.

(p3 <- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
  scale_colour_discrete(show_guide = FALSE) +
  geom_point()
)
# Error in discrete_scale

(p4 <- ggplot(mov, aes(year, rating, shape = mpaa)) +
  aes(colour = length, show_guide = FALSE) +
  geom_point()
)
#draws both legends

This question suggests that the modern (since ggplot2 v0.9.2) way of controlling legends is with the guides function.

I want to be able to do something like

p0 + guides(
  colour = guide_legend(show = FALSE) 
)

but guide_legend doesn’t have a show argument.

How do I specify which legends get displayed?