Hey so the question pretty much says it all, I've installed the irc client quassel and I want to get rid of it now but I don't know how to do so with pacman because I installed it from the source code, any help would be appreciated.
There is a script on the Arch Wiki pacman tips page for finding files not owned by any package. That will assist you tracking down what was installed with quassel.
In future, you should only ever use pacman to install packages, so they remain tracked and you don't end up cluttering your system with unmanaged files. Using ABS, the AUR or taking a little time to write your own PKGBUILD is the correct way to approach this.
You could force install qassel from the repos/aur, which should overwrite the source installed files and then remove the package, assuming stuff got installed to /usr and not /usr/local. For future reference, learn to write PKGBUILD buildscripts and use them to do from source installs, this results in a pacman package that you can install and remove more or less like repo packages.