Regarding your concern, this requirement is doable:
Step 1: Disable the feature for customers to checkout directly with PayPal. (when user buy a service from another user)
You can deactivate the MjE PayPal Express Checkout plugin in Plugins > Installed Plugins
Or go to Engine Setting > Payment Gateways > PayPal Express Checkout tab to disable this option
Step 2: Set Paypal payment gateway for your users to choose for package purchasing when posting new mJobs.
Please go to Engine Setting > Payment Gateways > General tab > 2. Default Payment Gateways > Paypal section and Enable this option.