By Lionel Laurent and George Georgiopoulos LONDON/ATHENS (Reuters) - Greece's cash crunch has pushed several European brokers to cut back on Greek share trading or restrict approval of new positions in case Athens imposes capital controls, industry sources told Reuters. The measures vary from broker to broker, rather than reflecting industry-wide moves. Trading sources in Athens said global banks were still routing Greek trades, while the Athens stock market operator said the proportion of turnover involving foreigners was little changed last month from a year earlier.