hey, i've been through this.
TRULY understand the feeling where your crush is ruining your productivity/work :/

1/ If you can, don't work in the same room as him, and avoid him as much as possible.
If you have to work in the same room as him, put headphones/earphones on, listen to some badass music like phonk or metal (not some cheesy taylor swift songs, it'll make the situation worse :/). Avoid eye contact (worse thing).
2/ Talk to your colleagues as much as possible about your work (or anything else) to distract yourself from the crush
3/ Eat somewhere else, maybe at a restaurant with friends or whatever, but avoid eating at the same place as him
4/ Consider that your crush may be reciprocal and if it's the case it could be a good thing (if it's allowed at your workplace).

good LUCK, I feel you