EXCLUSIVE

Charlie Sheen vows to hit Denise Richards where it really hurts -- in the child support department, because he says he's going to court to get a judge to slash the amount she gets.

Charlie is telling friends ... a judge has NEVER signed off on his current child support agreement with Denise -- it was all agreed to privately.

Denise currently gets $55,000 a month in support for their 2 kids. He's enraged that he's been so generous, and not just with child support. Charlie says he shelled out $9 million for the home he bought for her -- specifically so their 2 girls could be close to him.

So Charlie is telling his buddies ... Denise is epically ungrateful because he says she's denying him the right to have XMAS morning with the girls. He says she's actually taken away the girls' phones so they can't call him.