To access Greasemonkey's management pane, click on the downward arrow on the right side of the button Greasemonkey to the right of your URL bar and select Manage User Scripts. This opens a tab with a list of your scripts. For each script, you'll find a button to disable or uninstall it. There is also a Preferences button for each script, which lets you manually include or exclude pages for it to run on, but I doubt many people will tinker with this.
In addition to letting you manage your userscripts, the menu accessed from the Greasemonkey button lets you go to Greasemonkey options and add a new userscript (of your own devising). At the bottom, it also shows you which scripts are running on the page you are currently viewing.
While you can disable scripts individually, you can also disable all of them simply by clicking on the Greasemonkey button. With the monkey icon faded out, you know that all of your Greasemonkey scripts are disabled. Click on the button again to enable your scripts. To uninstall the Greasemonkey add-on itself, go to Tools > Add-ons and remove it.