First, please do not post in all-caps. If your caps-lock is stuck, replace your keyboard before you resume playing this game, thank you very much. An all-caps text is hard to read.
If you mean planet Infra, then
1. Manage Colonies;
2. Click the name of colony you wanna destroy Infra on;
3. in the Infra you would like to destroy, type in number of Infra points (labor assigned) to destroy;
4. Click Demolish.
If you mean researched tech Infra, you cannot destroy that.
You can always Plunder a planet or a few if you would like to drop below 5000 pr.