In your sentence, “is restrained” is correct. By itself, restraint is a noun. Straps binding a prisoner are restraints. No tense applies.
The verb, ‘restrain’ (present tense) is used as an imaginary restraint/strap: “Please restrain yourself” (stop yourself from doing whatever annoying action it is). Thus, a person is ‘restrained’ from doing something either by force, by contract