。
My parents forbade me from seeing Tom again.
Literal
Parents [topic-は] I [subject-が] again Tom [with-と] meet [nominalizer-こと] [object-を] prohibited.
~ことを禁止する (to prohibit doing X) nominalizes the clause and marks it as the object of 禁止する (to prohibit/ban). More formal and definitive than 許さない (not permit). また (again) implies they had met before.