Actually, YoungDave and ringkichard are correct about guards. There is nothing in the description of guard that says the guard must be a legal target for attacks in order to be effective. On the contrary, it makes the assertion that the guard is only ineffective in specific circumstances that have been explicitly spelled out, and being unable to legally attack the guard is not one of those circumstances.
Thus, putting Charm on the creature makes it unable to legally attack the Forcemaster. Then putting the Forcemaster on guard makes the creature unable to legally melee attack any other object within the Forcemaster's zone. Therefore, the creature could not attack anything within that zone at all and is very effectively neutered.
There is nothing in the rules that specifically mentions this exact interaction with Charm that I could find, however you can review the rules for Guarding on page 29 of the English rulebook v3.0 to see that it never mentions having to be able to attack the guard and instead mentions very specifically when you can ignore it. You can also review the FAQ page 47 regarding Suppression Cloak to see a very similar circumstance where you can not legally attack a guarding Wizard because you can not pay the mana cost necessary to, yet still can not ignore the protection it provides via guard resulting in no legal targets.
