Error Message prevents actions

Hi I activated the Global Footer by mistake and received the error message:

"The Global Footer Content plugin is only compatible with WordPress Multisite." (my site is not yet a multi-site)

Now, I cannot do anything else! Every time I try to deactivate the plug-in, or even close the site, it takes me back to this error message page. And yes, I did try to empty the cache, but that generates the same error message page ..

Getting dizzy ....