How to send a pop-up message to a particular user in SAP ?

User function module th_popup

Goto SM37 and execute the FM.By default the message is sent to the current system. If the RFC destinations are maintained it can be sent to any system in the landscape.

How are certian printers locked in QA and DEV environments..?

While running a production system, QA and DEV systems have access to the production printers also. In order to clog printers used in production , they can be locked from issuing outputs in different environments.
For example , printer US10 can be locked to in QA and DEV environments but only made active in PROD environment.

How to compare 2 ABAP programs in SAP..?

SE39

1. Two different/same programs can be compared
2. They can be compared across systems.
3. Different versions of the same/different programs can be compared