How to remove a particular portion from a plot?

It's hard to tell for sure without a reprex (short for reproducible example), as I'm not sure where plot_richness() comes from (it's not a ggplot2 function, I imagine it's a helper function from another package).

There's a nice FAQ on how to do a minimal reprex for beginners, below:

From the look of it, "Shannon" is the label for the facet of the plot (even though you don't actually have any other panels there, the default theming for facets have the grey strip right on top of the plot). So, you'd have to work with the strip.* theme arguments in order to change the appearance of that from its default label with plot_richness().

Depending on how plot_richness() works, I don't know whether or not you'll be able to modify the labeller() arguments.