There is no harm if the person needs to renew his wudhu' (ablution) that he puts in his place a prayer mat or something to reserve his place for himself. He has more right to his place. Verily, it has been reported in a hadith:
"If one of you gets up from his place of sitting, then returns to it, he has more right to it." (Muslim no.972)
Likewise, if he needs to recline and there is no place in the first row for him to recline against, he may place his prayer mat there and recline against a pillar after that. This is allowed as long as it does not cause hardship for the Muslims who come to pray, and he has the most right to his place. Reserving and saving a spot is only prohibited if the person reserves it early, then goes to his house or his shop for some worldly work or sleep or visiting or something similar. In a case like this, it is permissible to remove his item kept at the prayer place and offer prayer there.