Learn Italian: In questo museo l'accesso è severamente vietato. - In this museum, access is strictly forbidden.

You would say In questo museo l'accesso è severamente vietato a chiunque. You need the preposition a (to) because you are literally saying access is forbidden to anyone.

Why does Italian use l'accesso when English just says "access"?

Italian almost always requires a definite article before abstract nouns and general concepts. While English drops the article to just say "access", Italian needs it, literally saying "the access".

severamente translates to "strictly", but does it also mean "severely"?

Yes! The root adjective is severo, which can mean "severe" but is also the standard Italian word for "strict". By taking the feminine form (severa) and adding -mente, you get the adverb severamente (strictly).

Is vietato acting as a verb or an adjective here?

It is the past participle of the verb vietare (to forbid), but here it acts as an adjective describing l'accesso. Because accesso is a masculine singular noun, vietato must end in -o to match it.