How to unload a package without restarting R

Try this (see ?detach for more details):

detach("package:vegan", unload=TRUE)

---

It is possible to have multiple versions of a package loaded at once (for example, if you have a development version and a stable version in different libraries). To guarantee that all copies are detached, use this function.

detach_package <- function(pkg, character.only = FALSE)
{
  if(!character.only)
  {
    pkg <- deparse(substitute(pkg))
  }
  search_item <- paste("package", pkg, sep = ":")
  while(search_item %in% search())
  {
    detach(search_item, unload = TRUE, character.only = TRUE)
  }
}

Usage is, for example

detach_package(vegan)

or

detach_package("vegan", TRUE)

You can also use the unloadNamespace command, as in:

unloadNamespace("sqldf")

The function detaches the namespace prior to unloading it.

I tried what kohske wrote as an answer and I got error again, so I did some search and found this which worked for me (R 3.0.2):

require(splines) # package
detach(package:splines)

or also

library(splines)
pkg <- "package:splines"
detach(pkg, character.only = TRUE)

You can uncheck the checkbox button in RStudio (packages).