You can setup MW to auto delete a lot of your spam.
To auto delete senders you have already added to your blacklist for example, click Settings>>Spam Tools>>Blacklist>>Options, tick, "Auto delete blacklisted emails".
You can also set your filters, (Settings>>Spam Tools>>Filters>>click a filter>>Edit) to auto delete by selecting this option under the Actions tab there...
And to set MW to auto delete any email that comes in over a specific spam rating, click Settings>>Spam Tools>>Spam Ratings, and tick, "Auto delete if rating reaches:"
Note however, we recommend you read this first before enabling the last option: