First of all your material is protected through the copyright, but in order to collect any damages the copyright needs to be registered, so do that first. You should have a good argument because you have invoices they paid for training in the material so if they copy it fairly closely you could collect damages. A trademark isn't going to give you much protection on the material, trademarks are really only good for your brand and your logo, if part of your material is the brand then trademark it.

There are two other paths you can look at also. First ask the client about it, see if they are willing to partner in some way; for instance you could license your copyrighted material to them for use with their own clients. Second think about using a Creative Commons licence on the material that allows them to use it but drives their clients back to you; take a look at how Strategyzer has done this with the Business Model Generation Canvas.