Vehicles and creations that are either stolen/reuploaded content, contain profanity, or contain sexual content will be marked "incompatible" and will not be available to download/use. There are about half a dozen team members who regularly patrol the Workshop and take down anything they see.
What is stolen or what is a reupload?
Reuploaded items are items that have been downloaded from the Workshop (Or saved while in Multiplayer) and then uploaded to the Workshop with no or very little modifications without permission from the original creator.
Stoeln works are items that have been downloaded from the Workshop or saved while in Multiplayer and then modified before being uploaded. These still do not have permission and they will be taken down.
Profanity is not a specific thing. However, if the title or thumbnail contains profanity or the description contains a lot of profanity, then it will be removed.
What is sexual content? C'mon, use your brain here. Self explanitory.

How do I prevent these?
Profanity and Sexual content is a no-brainer. Make sure you only make and share content that you know parents will not get upset over.
For stolen and reuploaded works, just don't do it. You can, however, upload a modification of someone else's work if you have permission from the person and leave a link to the original build.

Also, please don't spam lots of stupid stuff, it'll likely get taken down as well.