from pymorphy2 import MorphAnalyzer
from pymorphy2.units.by_analogy import KnownPrefixAnalyzer, UnknownPrefixAnalyzer

morph = MorphAnalyzer()

def cut_prefix(morph, word):
    analyzers = [
        KnownPrefixAnalyzer(morph),
        UnknownPrefixAnalyzer(morph)]
    seen = set()
    word_lower = word.lower()
    res = []
    for analyzer in analyzers:
        parsed = analyzer.parse(word, word_lower, seen)
        if not parsed:
            continue
        tmp = parsed[0]
        res.extend(tmp)
        if len(tmp) >= 5:
            if tmp[4] and tmp[4][0]:
                return morph.normal_forms(tmp[4][0][1])[0]
    return res

In [126]: cut_prefix(morph, "приукрасила")
Out[126]: 'украсить'

In [127]: cut_prefix(morph, "спортлото")
Out[127]: 'лото'

In [128]: cut_prefix(morph, "мегаабракадабра")
Out[128]: 'абракадабра'

In [129]: cut_prefix(morph, "подловил")
Out[129]: 'ловить'