Bottom line, there is no good encryption that works well.
Most encryption systems cause problems actually working, and none of them even work very well. The people who do not know how to decrypt it do not have to, only one person has to know how and post it somewhere. If you do not want your resources to be open for public viewing, then using a different engine is really your only option. RPG Maker pretty much can never be protected in the way that you are looking for, and as gameus said, it is not like anyone can/will ever actually use them. Your target users are the very people you are concerned about stealing them, but they could never really do that, because there target users would be the same, and those users would notice that, or you yourself would.