23 July 1970
In 1963, the Security Council called upon states to stop the sale of arms to South Africa, and repeated the call in 1970 and again in 1972. It was however only in 1977 that the arms embargo became mandatory, and other sanctions were still not in place.  On 23 July 1970, the United Nations (UN) Security Council condemned all violations of its embargo against South Africa. After five meetings on the issue, Resolution 281 (1970) was subsequently passed calling on all states to strengthen the arms embargo against South Africa. The resolution was adopted by twelve votes to none against, with France, Great Britain and the United States abstaining.
References

South African History Online, ‘South Africa and the United Nations, 1946-1990’, [online], available at www.sahistory.org.za(Accessed: 20 June 2013)|

O’Malley, P. ‘1970’, from Nelson Mandela Centre of Memory, [online], available at www.nelsonmandela.org.za(Accessed: 20 June 2013)