The forward slash character is a special one in webserver country. Apache rejects %2F characters outright, while IIS (6 - 7.5) always translates it to an actual slash.
If you cannot convince editors to replace the forward slash with something else (which is by far the best thing you can do), then you could do two things:
For importing content through content porter you can write a tool to first translate all Tridion item titles which contain a slash to some token. Import the items in the target system and then run the tool again to rename the items back. By doing this, webdav will still not work though, but at least you get the items imported correctly.
Another option is way more tricky: use IIS rewriting to translate all %2F sequences to %252F sequences. IIS Rewrite can't do this out of the box, so you'll have to be creative. Further, you have to configure allowDoubleEscaping in IIS for it to work, but that is potentially dangerous.
So all in all - the best thing to do is to not have forward slashes in item titles.
I would further like to note that this hasn't got anything to do with Content Porter, it's just the way IIS handles forward slashes.
HTTP 400 - Bad request
errors.