How do you block a satellite?

Answer

Dear Simon,
you may be asking the question in response to the recent collision between two satellites.
Satellites are placed in their orbit around the Earth by means of a launcher. Once they have been placed in the desired orbit, that launcher is disconnected. The satellite usually has little or no possibilities to change its orbit. It can rotate around its axes, for example to make observations in a different direction.
Therefore, the orbits of all satellites, both the active and the out-of-service satellites, are closely monitored. When a new satellite is launched, the orbit is chosen in such a way that no collisions will occur. It actually succeeds very well, because the recent collision is the first in decades of space travel. (unless the Russians and Americans have not told us something…)
Satellites that have been taken out of service are usually left flying around. Very slowly their orbits sink as a result of the friction with the outermost layers of the atmosphere, and after a number of years they dive into the atmosphere where they usually burn up completely. Really large satellites like the old space station MIR are released into the atmosphere in a controlled way, because there are debris that could fall to Earth anyway. They are therefore allowed to dive into the atmosphere in a place where the risk is the smallest, for example above the south Pacific Ocean.
What you should definitely not do is shoot up old satellites. After all, instead of one large satellite whose orbit you know well, you get hundreds of small pieces in all kinds of orbits. Each of those hundreds of pieces then becomes a dangerous projectile for the satellites in nearby orbits.