Can not cancel something that is build into the design.
What is made by one human can be unmade by another human (and most certainly will be). Personally, I wouldn't count on this...
Laws carved in stone are rewritten in blood
The miners can just modify the reward as they feel fit.
Unfortunately, for them, the blocks with a different reward will be considered invalid and rejected by the full nodes.
They can mine as much as they are able, but the rules on the other full nodes will not change and their blocks will be discarded.
For example, the exchanges will not accept the bitcoin mined by invalid blocks. The shops will not accept them.
Now, the Core developers could change the rules in a new version of the Core, but I would not count on the users to download and upgrade their nodes with the new code when they discover what is in it.
If Bitcoin halving will negatively affect its infrastructure, up to a point of an escalating failure (e.g. due to miners leaving), this may be the only option, that is, to change the protocol and cancel the halving...