For the time being, this feature will be just like the Google +1 or Facebook "Like", whereby if a user likes content, they can unlike said content at any time (as you're showing appreciation to the author of a posting for their content; if they're having a spat, it's not this mod's fault, it didn't do anything wrong, it isn't deserving of a spanking). I just don't feel comfortable with saying "you like this now, but a month from now you might not, and there might be outside factors, but you're stuck liking this forever", that's pretty much against a "like" system.
If you want to modify the feature to make it so that users cannot "unlike" that'd be a choice for you (simply modify the "down" area of like.inc.php), but making the software anything more or less than the current "real-world" systems it's being modeled after isn't the goal of what's going to be provided thus far.