Copyright of Rebus puzzles

[Castle2]

A few months ago I created an iPhone game based on rebus puzzles. These puzzles are things like:

yyy men

arrest
UR

must get here
must get here
must get here

The solutions are "Three wise men", "You are under arrest" and "Three Musketeers".

Now, I may have infringed upon a copyright by merely giving these examples. I was contacted by someone claiming to own the copyright to the Dingbats brand. Details can be provided if requested. They claimed that a significant number of the puzzles in my game duplicated their copyright material. I was surprised to learn this, since I spent a lot of time creating the puzzles, but they provided examples of a number of the puzzles in their game and my corresponding puzzles.

For most of them there was clearly a significant similarity. If these type of puzzles can be copyrighted, then I have clearly (unintentionally) infringed with some of them. I immediately removed my game from distribution and conceded to all the demands of the claimant, since I have no desire to get sued. However, a number of the puzzles they claimed I had infringed upon are quite different.

For example, one was for the phrase "Treasure Island". I had used the word "Treasure" and then a picture of an island. In their example, the word "Treasure" was written to look like an island. I can provide images in needed, but won't post them in case doing so infringes upon their copyright. Now, to my eyes these two are significantly different. They are not visually the same, the pictures aren't the same, the ways of solving the puzzle are different and the method of constructing the puzzle is different. I requested clarification from the claimant as to the exact reasons why they considered that puzzle to be infringing. They have ignored several requests for such clarification.

My first question, then, is can such puzzles be copyrighted?

If so, my second question is how similar they must be to count as infringing? In some cases I'd used the same logic to create the puzzle, but totally different words to construct it. Again, examples can be provided. I'm not interested particularly in contesting these examples, but it has major implications for my creation of future puzzles.

Finally, they have as yet provided no evidence that they are the copyright holder, saying that I should research it myself. Is the onus on the one making the accusation of copyright infringement or the one being accused? I've done some searching and can't find any registration of copyright that he claims to have.

I am happy to clarify any points or add any more details, and full expect to be contacted by the claimant to demand the removal of this post (as he did when I discussed the threat in general terms on a Mac forum).

Many thanks for any help!