Some weird and inaccurate statements on here!
Cats do have the right to roam. That's fact. If you don't like it then your avenue of redress is probably petitioning parliament or starting some kind of campaign, not taking it upon yourself to start mistreating cats (who, by the way, are not doing anything wrong).
It is against the law to trap a cat. If you took it to a shelter because it ventured over to your garden, despite knowing whose pet it is and that it isn't being mistreated, they would think you were batshit. You would also be wasting a charity's time.
Cat owners generally care about the welfare of their pets. This includes letting them roam freely and engage in their natural behaviours (their right to which is also protected by law). Most cat owners are fine with them being shooed, scared away, squirted with water, deterred with lavender or any other sane, normal methods. Cat owners will generally not be ok with you trapping or harming them.
Incidentally, cats are actually legally considered the property of the owner. So if you harm a person's cat you have actually caused damage to their property. So are you any better?
There are vast amounts of animals that could be defecating in your garden, other than cats. Have a long think about it and ask yourself whether you're getting so annoyed simply because you've found a scapegoat. Imagine it's a fox. What would you do then?
I speak as a cat owner who has taken steps to make sure my cats don't go over the fence on one side, I have fully supported my neighbour shooing them away, and I have also thrown water on other cats. I am not blind to the impact my cats have on others, but I also think people need to be a lot more tolerant.