No, that's not quite right.
WotC Compendium said:
* If a Pokemon has copies of other Pokemon's attacks on it (such as Sabrina's Alakazam), it will NOT have multiple copies of identical attacks. This means that is Sabrina's Alakazam was your Active Pokemon and you had a Mind Shock attack that does 30 damage and a Mind Shock attack that does 40 damage on the bench, you would gain a copy of each of those attacks as they are NOT identical (the amount of damage done is different). Different attacks can have the same name. An attack is considered identical if it has the same attack cost, name, and does identical damage and effects. If Amnesia ("Choose 1 of your opponent's attacks, that attack cannot be used next turn") is done against the Sabrina's Alakazam in this example, the attacker would have to choose the Mind Shock that does 30 damage or the Mind Shock that does 40 damage, NOT just "Mind Shock". (Feb 14, 2002 WotC Chat, Q18)
Therefore, if Unown M's Hidden Power is Disabled/Amnesia'd, and another Unown is Shuffled into play, you would be able to use that Hidden Power
unless another Unown M was brought into play, since all Hidden Power attacks are different.
For m0nster1nc's example, the quote above explains it all. You can't choose "Slash", but one of the copies of "Slash" in play. If a Slash attack did 40 damage and another one did 50, you would need to pick a Slash attack to disable.
However, if you disabled Power Move on Mew ex, then Mew ex would not be able to copy another Power Move because the attack is identical to the one disabled.
So, in summation:
Unown CAN use Hidden Power as long as it doesn't have the same effect as the one disabled,
Two attacks with the same name are not disabled simultaneously, and
if an attack is disabled, all other EXACT copies of that attack (attack cost, name, damage and effects) are disabled as well.
Hope this clears things up.