If the paint is water based try hot water and washing up liquid, in conjunction with a medium bristle brush like a shoe polishing brush or with a soft kitchen scourer, gently.
If the paint is oil based, try white spirit or turpentine, apply with a paint brush, leave a bit, gently wipe with soft scourer, repeat as needed, wash off with soapy water.
Test the white spirit/turpentine first on an inconspicuous bit of plastic, just in case.