In current versions of matplotlib you can do:
boxplot([1,2,3,4,5,10], showfliers=False)
or
boxplot([1,2,3,4,5,10], sym='')
In older versions, only the second approach will work.
The docs for boxplot do mention this, btw as, “Enter an empty string (‘’) if you don’t want to show fliers.”, though, at least for myself, “outliers” is the more familiar word.