[R] grid - deleting and erasing grobs?

[M.Kondrin]

Hello!
Don't quite understand how can I delete grobs and simultaneously erase 
graphic output they produce. I first change grob's "vp" field to null 
(grid.edit(gr,vp=NULL)) to erase it and then call rm(gr) (as grobs are 
external pointers I'm not shure what this method actually frees 
allocated memory).
May be there is simpler method?
Does garbage collector have any effect on grobs?
Thank you in advance