V brakes either too weak or rub
One problem that leads to this exact symptom is if someone installed V-brakes along with cantilever/caliper brake levers, or in the case of a multi-setting brake lever, has it set to the "short pull" mode.

You can get more detail on how to tell the difference from this question.

If it's an incompatibility between levers and brakes, it's usually going to be cheaper parts-wise to replace the levers. There's not really another fix.