x = rand(10,3);
boxplot(x);
Normally (at least for most other plot functions) you could do this sort of thing with a name-value pair argument or by getting the handle for example with h = boxplot(x) and then modifying the properties there. However, I didn't see any options to remove or hide the red line with those approaches. So I suggest using findobj as follows:
figure(), boxplot(x)
h = findobj(gca,'Tag','Median');
delete(h)
