Thanks for the advice, heiska!
What I came up with yesterday:
1. Just allow a-z A-Z 0-9 , ! . ? -
If there's any other character in it, it's not a sentence! This will filter out some correct sentences but works quite good...
2. Check for the length and the number of spaces in it.
3. Only grab content between p-html-tags!
The results are pretty good now...