Your code constantly updates because nPts is local to a module enclosing the Manipulate. If the Manipulate enclosed the module, your code wouldn't work as you would like, but it wouldn't be constantly updating.
Here is a much simpler example showing the same behavior.
Module[{n = 1}, Manipulate[Row[{"n = ", n++}], {t, 0, 5}]]
constantly updates, but
Manipulate[Module[{n = 1}, Row[{"n = ", n++}]], {t, 0, 5}]
doesn't.
I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button
Manipulate[
If[update,
update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
ClickPane[
Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}],
AppendTo[pts, #] &],
{{update, False}, ControlType -> None},
{{pts, {{0, 0}}}, ControlType -> None},
Button["Update", update = True]]
This is much simpler than your code, but still performs the simulation you trying to implement. Give it a try and see if it satisfies your needs.