blockieren
to block
To physically block a path, road, or entrance so that no one can pass through.
Ein umgestürzter Baum blockiert die gesamte Fahrbahn.
A fallen tree is blocking the entire roadway.

to block
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to block
To physically block a path, road, or entrance so that no one can pass through.
Ein umgestürzter Baum blockiert die gesamte Fahrbahn.
A fallen tree is blocking the entire roadway.

'Blockieren' is a verb ending in '-ieren' that comes from an international word stem. These verbs are always regular, not separable, and form the past participle with '-iert' ('blockiert').
Er hat den Parkplatz für seinen Chef blockiert.
He blocked the parking space for his boss.
The meaning of 'blockieren' becomes clear from the object. Physical things like 'Straße' (street) or 'Tür' (door) are blocked. Abstract things like 'Gesetz' (law) or 'Prozess' (process) are prevented. Appointments or resources are reserved.
Die Demonstranten blockierten die Brücke für mehrere Stunden.
The demonstrators blocked the bridge for several hours.
Only in the meaning of being mentally inhibited is 'blockieren' used without an accusative object. It then describes the state of the person who is acting—or rather, cannot act.
Ich war so nervös, dass ich bei der mündlichen Prüfung komplett blockiert habe.
I was so nervous that I completely froze up during the oral exam.
The meaning 'reservieren' (to reserve) is mainly used in colloquial language or everyday office contexts. It is an informal way to express that you are setting something aside for yourself or others.
Kannst du den Nachmittag in meinem Kalender für unser Projekt blockieren?
Can you block the afternoon in my calendar for our project?
to prevent
'verhindern' and 'blockieren' can both mean that something cannot happen or cannot continue. 'verhindern' is more general: you make sure that an event, a plan, or an action does not happen. 'blockieren' sounds more concrete and often stronger: something is obstructed, stopped, or disrupted in its function. You can block a road, but also a process or a decision. Learners may confuse the words because both express stopping something. As a simple guide: 'verhindern' fits the result that something does not happen; 'blockieren' fits more when something is actively in the way or holds up a process.

Ein Lieferwagen blockiert die Einfahrt zum Supermarkt.
A delivery van is blocking the entrance to the supermarket.
Die schnelle Hilfe verhindert einen größeren Schaden.
The quick assistance prevents greater damage.