What is the procedure to cancel Vodafone sim and internet service. I need to do the cancellation since I am leaving the Czech Republic and I no longer need the service.

I have already returned to my home country and hence it is not possible to go to the Vodafone office in person.

Hi, officially:

The cancellation process primarily relies on submitting a written request (a termination form) to the Vodafone headquarters.

Fill Termination Form
Vodafone requires a specific form for contract termination, especially for cancelling fixed-term agreements or services with favorable conditions.
vypoved_smlouvy_o_poskytovani_zvyhodnenych_podminek_c.pdf (specimen in English)

Strongly recommend sending it by Registered Mail (Doporučený dopis) to ensure you have proof of delivery.

Address:
Vodafone Czech Republic a.s.

náměstí Junkových 2

155 00 Praha 5

Czech Republic

Vodafone will typically confirm the receipt of your termination notice via SMS.
The standard notice period is 30 days. This period begins running from the day the written notice is officially received at the Vodafone headquarters.
If you have rented any equipment from Vodafone (like a modem or set-top box for fixed internet/TV), you must return it within 14 days after the services are officially terminated.

So... call to Vodafone and try to talk it over with them. Maybe you will find with them alternative way in your situation.

You can reach Vodafone from abroad at +420 776 977 100.