Spraying mealy bugs with regular pesticide will not because mealy bugs fuzzy like protection with keep the mealy bug safe.

The way to kill and smother mealy bugs is with neem oil.

Another way to kill mealy bugs is with regular soupy spray.

If you find mealy bugs in your greenhouse or garden you should, through the plant away, if possible, cut the parts of the plant that are affected, and you could spray neem oil regularly to ever prevent it.