Wall of Fire not so fiery apparently.

[Galabranth]

 {Wall of Fire}

Pay R to get +1/+0 until end of turn. Now my friend is arguing that Wall of Fire's ability can be stacked, meaning he can repeatedly pay the mana cost to increase its Power.

E.G. On my turn he has 7 untapped Mountains. As I declare my now +5/+5 Centaur as an attacker, he pays 7 mana to increase Wall of Fire's power to +7. He blocks my centaur and puts my centaur in the graveyard. Is this legal?

He won't believe me when I say it isn't so I'm turning to you guys.

[Anoobass]

{Wall of Fire}

Pay R to get +1/+0 until end of turn. Now my friend is arguing that Wall of Fire's ability can be stacked, meaning he can repeatedly pay the mana cost to increase its Power.

E.G. On my turn he has 7 untapped Mountains. As I declare my now +5/+5 Centaur as an attacker, he pays 7 mana to increase Wall of Fire's power to +7. He blocks my centaur and puts my centaur in the graveyard. Is this legal?

He won't believe me when I say it isn't so I'm turning to you guys.

I'm sorry but he is right, he can pay for it as many times as he likes, that is what makes these cards good in red decks.  If you have nothing to cast, there's always boosting a creature.  The only card like this that I know of with a "limitation" is {dragon whelp}.  And even then, he could still do it as many times above 3 as he wanted...once...

[Galabranth]

{Wall of Fire}

Pay R to get +1/+0 until end of turn. Now my friend is arguing that Wall of Fire's ability can be stacked, meaning he can repeatedly pay the mana cost to increase its Power.

E.G. On my turn he has 7 untapped Mountains. As I declare my now +5/+5 Centaur as an attacker, he pays 7 mana to increase Wall of Fire's power to +7. He blocks my centaur and puts my centaur in the graveyard. Is this legal?

He won't believe me when I say it isn't so I'm turning to you guys.

I'm sorry but he is right, he can pay for it as many times as he likes, that is what makes these cards good in red decks.  If you have nothing to cast, there's always boosting a creature.  The only card like this that I know of with a "limitation" is {dragon whelp}.  And even then, he could still do it as many times above 3 as he wanted...once...

Just goes to say one should never assume too much in magic. Thanks! I think I've just been burned haha. (Does that mean I cards with abilities paid with mana can be used multiple times? e.g. {Vitu-Ghazi Guildmage} 's Populate?)

[Kaworu, the Fifth Child]

{Wall of Fire}

Pay R to get +1/+0 until end of turn. Now my friend is arguing that Wall of Fire's ability can be stacked, meaning he can repeatedly pay the mana cost to increase its Power.

E.G. On my turn he has 7 untapped Mountains. As I declare my now +5/+5 Centaur as an attacker, he pays 7 mana to increase Wall of Fire's power to +7. He blocks my centaur and puts my centaur in the graveyard. Is this legal?

He won't believe me when I say it isn't so I'm turning to you guys.

I'm sorry but he is right, he can pay for it as many times as he likes, that is what makes these cards good in red decks.  If you have nothing to cast, there's always boosting a creature.  The only card like this that I know of with a "limitation" is {dragon whelp}.  And even then, he could still do it as many times above 3 as he wanted...once...

Just goes to say one should never assume too much in magic. Thanks! I think I've just been burned haha. (Does that mean I cards with abilities paid with mana can be used multiple times? e.g. {Vitu-Ghazi Guildmage} 's Populate?)

Unless something says otherwise yes.

[Destore117]

All mana paid abilities that don't use the: {T} symbol can be used more than once unless otherwise stated.

[Anoobass]

All mana paid abilities that don't use the: {T} symbol can be used more than once unless otherwise stated.

And even if they have the {T} as a cost, and you untap it somehow, you can use the ability again.  So long as you can pay the cost (everything before the colon) you can get the effect.