I don't know R at all, but a bit of creative googling led me here: http://tolstoy.newcastle.edu.au/R/help/05/04/1919.html
The key quote from there:
I do not find explicit documentation for R on how to remove elements from lists, but trial and error tells me
myList[] <- NULL
will remove the 5th element and then "close up" the hole caused by deletion of that element. That suffles the index values, So I have to be careful in dropping elements. I must work from the back of the list to the front.
For deleting an element of a list, see R FAQ 7.1
And the relevant section of the R FAQ says:
... Do not set x[i] or x[[i]] to NULL, because this will remove the corresponding component from the list.
Which seems to tell you (in a somewhat backwards way) how to remove an element.
Hope that helps, or at least leads you in the right direction.