Why would it be new stuff? Afaik there already are characters out there, who can prevent healing. Galilea can do that via upgraded Desecrate and Shield Throw at early stages. Her effects last for 5 seconds each.
I would give WT the same debuff via toxic bullets, but with only a 2 second duration, which can be refreshed by consecutive hits.
That would serve 2 purposes. First it would impact his damage output without upping his damage. His damage is not this bad, seen in PvE. The problem is, that he has no true burst damage abilities, so his damage can easily be predicted and healed off. Not so with this little extra. Secondly he would become a serious counter to heal support badasses like Ambra, Miko, etc.
About that neurotoxin and the inverted controls, while I think this would be more fun, a proper 2-3 seconds desorientation might work as well.