If someone has been sentenced for any offence, this will automatically disqualify them. This includes suspended sentences where the person hasn't actually served time in prison
A judge may discharge a juror due to lack of capacity. This includes not being able to understand English or having a disability which stops them from performing their duties
In 1995, a deaf man was prevented from sitting on a jury. He wished to bring a sign language interpreter but this was not allowed as the judge pointed out that an extra person would be required in the courtroom, which is against court rules