Pop up box won't go away

I was staying in a hotel and used the free wifi, however it asked if I wanted to upgrade the wifi and declined. I am now getting a pop up box saying “UpgradeFilterfld22318073349” will damage your computer.

It gives two options click ok or show in finder, if you click ok it reappears, if you click show in finder it doesn't come up. However I did a search and it found a document which I deleted and then emptied the trash but it is still appearing after a restart, not sure how to get rid of it now. I can still use my MAC but have the move the box out of the way