Laughter is often the best medicine. With entities like Slenders who feed on negative energy, sometimes the best defense is changing your mood. Since you can't poke him in the eye, do the next best thing and laugh in his face.
One woman told me that Slenders was visiting her in her dreams and would simply stare at her as if waiting for her to say or do something. She started asking him questions and every time he didn't answer, she slapped a sticker on his pallid bald head. By the time he left, he was covered with the things, and she did it every time he visited until he stopped.
Whatever works for you. So long as it lightens your mood, it's all good.
Here's some Slender Parody in no particular order:
Phenomena- Slender Parody
MarbleHornets Parody 1, 2, 3, 4, 5, 6
Gimme 20 Dollars
Slenderman's Favorite Song
Time to Rave
Why Are You Running?
And some other stuff:
Zeke Strahm "facts"
DAS Archived Slender-posts
Does Whatever a... Slender can?
Fun Things (Webcomics)
The Show with Ze Frank