You can include in your chunk options something like
comment=NA # to remove all hashes
or
comment="%" # to use a different character
More help on knitr available from here: http://yihui.name/knitr/options
If you are using R Markdown as you mentioned, your chunk could look like this:
```{r comment=NA}
summary(cars)
```
If you want to change this globally, you can include a chunk in your document:
```{r include=FALSE}
knitr::opts_chunk$set(comment = NA)
```